Add version compatibility policy guidelines for Cirq (quantumlib#5897)

Cirq is now officially at v1.0 and we need an official policy for describing guidelines for version compatibility across cirq versions. 

This PR adds a document describing the new official policy. Please take a look and leave comments / give feedback. I'd be very interested to hear about use cases which are potentially unclear from the policy so we can make any necessary improvements.

cc @mpharrigan @dstrain115

Author: tanujkhattar

Diff for: docs/_book.yaml

@@ -237,6 +237,8 @@ upper_tabs:
         path: /cirq/dev/modules
       - title: "Serialization guidelines"
         path: /cirq/dev/serialization
+      - title: "Version compatibility"
+        path: /cirq/dev/versions
       - title: "Triage process"
         path: /cirq/dev/triage
       - title: "Issues / requests / questions"

Diff for: docs/dev/versions.md

@@ -0,0 +1,44 @@
+# Cirq version compatibility
+
+## Overview
+This document defines backwards compatibility guidelines across different versions of Cirq (either for code or data).
+
+## Semantic Versioning 2.0
+Cirq follows Semantic Versioning 2.0 ([semver](http://semver.org/)) for its public API. Each release version of Cirq has the form MAJOR.MINOR.PATCH. Changes to each number have the following meaning:
+
+*   **MAJOR**: Potentially backwards incompatible changes. Code and data that worked with a previous major release will not necessarily work with the new release. However, in most cases existing json serialized Cirq objects would be migratable to the newer release; see [Compatibility of serialized objects](#compatibility-of-serialized-objects) for details on data compatibility.
+*   **MINOR**: Backwards compatible features, speed improvements, etc. Code and data that worked with a previous minor release _and_ which depends only on the non-experimental public API will continue to work unchanged. For details on what is and is not the public API, see [What is covered](#what-is-covered).
+*   **PATCH**: Backwards compatible bug fixes.
+
+## What is covered
+The public APIs of Cirq are backwards compatible across minor and patch versions. The public APIs consist of
+
+* All the documented [symbols](https://quantumai.google/reference/python/cirq/all_symbols) (module, function, argument, property, class, or constant) in the Cirq module and its submodules, except for
+    *   Private symbols: any symbol whose name start with an underscore **\_**.
+    *   Experimental and `cirq.contrib` symbols, see [below](#what-is-not-covered) for details.
+* Note that only code reachable through the `cirq` Python module is covered by the compatibility guarantees. 
+    *   For example: Code in the [examples/](https://github.com/quantumlib/Cirq/tree/master/examples) and [dev\_tools/](https://github.com/quantumlib/Cirq/tree/master/dev_tools) directories is not reachable through the `cirq` Python module and is thus not covered by the compatibility guarantee. 
+    *   Similarly, symbols in vendor packages, like [cirq-google](https://quantumai.google/reference/python/cirq_google/all_symbols), [cirq-aqt](https://quantumai.google/reference/python/cirq_aqt/all_symbols) are also not covered by the compatibility guarantee.
+    *   If a symbol is available through the `cirq` Python module or its submodules, but is not documented, then it is **not** considered part of the public API.
+
+## What is _not_ covered
+* **Experimental APIs**: To facilitate development, we exempt some API symbols clearly marked as experimental from the compatibility guarantees. In particular, the following are not covered by any compatibility guarantees:
+    *   any symbol in the `cirq.contrib` module or its submodules.
+    *   any symbol whose name contains `experimental` or `Experimental`; or
+    *   any symbol whose fully qualified name includes a module or class which is itself experimental. This includes fields and submessages of any json/protocol serialization called `experimental`.
+* **Details of gate decompositions:** Many public gates and operations in Cirq are composite gates defined by specifying a particular decomposition in terms of other simpler gates, using `_decompose_` protocol. These decompositions may change for minor releases, with a guarantee that the old and new decompositions would result in the same unitary up to a global phase.
+* **Floating point numerical details:** The specific floating point values computed by simulators / linalg utilities may change at any time. Users should rely only on approximate accuracy and numerical stability, not on the specific bits computed.
+* **Type Preservation:** The exact type of the objects consumed and returned by methods / functions in the public API can be changed any time, as long as the change satisfies [Liskov Substitution Principle (LSP)](https://en.wikipedia.org/wiki/Liskov_substitution_principle).
+* **Bugs:** We reserve the right to make backwards incompatible behavior (though not API) changes if the current implementation is clearly broken, that is, if it contradicts the documentation or if a well-known and well-defined intended behavior is not properly implemented due to a bug.
+    *   For example: if a transformer claims to implement a well-known transformation algorithm but does not match that algorithm due to a bug, then we will fix the transformer. Our fix may break code relying on the wrong behavior of the transformer. We will note such changes in the release notes.
+* **Circuit diagrams:** We reserve the right to change exact diagrams for gates / operations / circuits across minor releases. These changes would also be explicitly announced on cirq-announce@.
+    *   For example: We can change [SVG diagrams](https://github.com/quantumlib/Cirq/issues/5689) and [text diagrams](https://github.com/quantumlib/Cirq/issues/5688) to use a different symbol for all classically controlled operations.
+* **Error behavior:** We may replace input validation errors with non-error behavior. For instance, we may change a class/function to compute a result instead of raising an error for a certain set of inputs, even if that error is documented. We also reserve the right to change the text of error messages.
+    *   For example: If we decide to extend functionality of `PauliSumExponential` to support non-commuting Pauli’s, the [value error](https://github.com/quantumlib/Cirq/blob/e00767a2ef1233e82e9089cf3801a77e4cc3aea3/cirq-core/cirq/ops/pauli_sum_exponential.py#L53) in the constructor would be removed.
+
+## Compatibility of serialized objects
+Cirq currently uses JSON serialization for serializing gates, operations and circuits. Many Cirq users serialize and store their circuits so they can be loaded and executed with a later release of Cirq. In compliance with [semver](https://semver.org/), serialized Json objects written with one version of Cirq can be loaded and evaluated with a later version of Cirq with the same major release.
+
+We make additional guarantees for _supported_ Cirq serialized objects. A serialized object which was created using **only non-deprecated, non-experimental APIs** in Cirq major version `N` is a _supported serialization in version `N`_. Any serialized object supported in Cirq major version `N` can be loaded and executed with Cirq major version `N+1`. However, the functionality required to build or modify such an object may not be available anymore, so this guarantee only applies to the unmodified serialized objects.
+
+We will endeavor to preserve backwards compatibility as long as possible, so that the serialized objects are usable over long periods of time.