CONTRIBUTING: add a note about instrumentation versioning and depdendencies (open-telemetry#2851)

Author: xrmx

Diff for: CONTRIBUTING.md

@@ -275,6 +275,7 @@ Below is a checklist of things to be mindful of when implementing a new instrume
 - Isolate sync and async test
   - For synchronous tests, the typical test case class is inherited from `opentelemetry.test.test_base.TestBase`. However, if you want to write asynchronous tests, the test case class should inherit also from `IsolatedAsyncioTestCase`. Adding asynchronous tests to a common test class can lead to tests passing without actually running, which can be misleading.
   - ex. <https://github.com/open-telemetry/opentelemetry-python-contrib/blob/60fb936b7e5371b3e5587074906c49fb873cbd76/instrumentation/opentelemetry-instrumentation-grpc/tests/test_aio_server_interceptor.py#L84>
+- All instrumentations have the same version. If you are going to develop a new instrumentation it would probably have `X.Y.dev` version and depends on `opentelemetry-instrumentation` and `opentelemetry-semantic-conventions` for the same version. That means that if you want to install your instrumentation you need to install its dependencies from this repo and the core repo also from git.
 
 ## Expectations from contributors