Change master to main in documentation (#6365)

- We are changing the default branch from master to main
in order to conform to best practices and remove problematic
terminology.
- This PR changes references in documentation (mostly links)
to use main.

Author: dstrain115

CONTRIBUTING.md

@@ -38,27 +38,27 @@ Forking creates a new github repo at the location
 your github id. Use the directions on the
 [development page](docs/dev/development.md) to download a copy to
 your local machine. You need only do this once.
-1. Checkout master and create a new branch from this master
+1. Checkout main and create a new branch from this main
     ```shell
-    git checkout master -b new_branch_name
+    git checkout main -b new_branch_name
     ```
     where ```new_branch_name``` is the name of your new branch.
 1. Do your work and commit your changes to this branch.
-1. If you have drifted out of sync with the master from the
+1. If you have drifted out of sync with the main from the
 main cirq repo you may need to merge in changes.  To do this,
-first update your local master and then merge the local master
+first update your local main and then merge the local main
 into your branch:
     ```shell
     # Track the upstream repo (if your local repo hasn't):
     git remote add upstream https://github.com/quantumlib/Cirq.git
 
-    # Update your local master.
+    # Update your local main.
     git fetch upstream
-    git checkout master
-    git merge upstream/master
-    # Merge local master into your branch.
+    git checkout main
+    git merge upstream/main
+    # Merge local main into your branch.
     git checkout new_branch_name
-    git merge master
+    git merge main
     ```
     You may need to fix merge conflicts for both of these merge
     commands.

README.rst

@@ -1,4 +1,4 @@
-.. image:: https://raw.githubusercontent.com/quantumlib/Cirq/master/docs/images/Cirq_logo_color.png
+.. image:: https://raw.githubusercontent.com/quantumlib/Cirq/main/docs/images/Cirq_logo_color.png
   :target: https://github.com/quantumlib/cirq
   :alt: Cirq
   :width: 500px
@@ -10,7 +10,7 @@ circuits and running them against quantum computers and simulators.
   :target: https://github.com/quantumlib/Cirq
   :alt: Build Status
 
-.. image:: https://codecov.io/gh/quantumlib/Cirq/branch/master/graph/badge.svg
+.. image:: https://codecov.io/gh/quantumlib/Cirq/branch/main/graph/badge.svg
   :target: https://codecov.io/gh/quantumlib/Cirq
 
 .. image:: https://badge.fury.io/py/cirq.svg
@@ -26,7 +26,7 @@ Installation and Documentation
 
 Cirq documentation is available at `quantumai.google/cirq <https://quantumai.google/cirq>`_.
 
-Documentation for the latest **pre-release** version of cirq (tracks the repository's master branch; what you get if you ``pip install --pre cirq``), is available `here <https://quantumai.google/reference/python/cirq/all_symbols?version=nightly>`__.
+Documentation for the latest **pre-release** version of cirq (tracks the repository's main branch; what you get if you ``pip install --pre cirq``), is available `here <https://quantumai.google/reference/python/cirq/all_symbols?version=nightly>`__.
 
 Documentation for the latest **stable** version of cirq (what you get if you ``pip install cirq``) is available `here <https://quantumai.google/reference/python/cirq/all_symbols>`__.
 
@@ -98,10 +98,10 @@ Cirq Contributors Community
 ---------------------------
 
 We welcome contributions! Before opening your first PR, a good place to start is to read our
-`guidelines <https://github.com/quantumlib/cirq/blob/master/CONTRIBUTING.md>`__.
+`guidelines <https://github.com/quantumlib/cirq/blob/main/CONTRIBUTING.md>`__.
 
 We are dedicated to cultivating an open and inclusive community to build software for near term quantum computers.
-Please read our `code of conduct <https://github.com/quantumlib/cirq/blob/master/CODE_OF_CONDUCT.md>`__ for the rules of engagement within our community.
+Please read our `code of conduct <https://github.com/quantumlib/cirq/blob/main/CODE_OF_CONDUCT.md>`__ for the rules of engagement within our community.
 
 For real time informal discussions about Cirq, join our `cirqdev <https://gitter.im/cirqdev/community>`__ Gitter channel, come hangout with us!