Create versioning and release policy document (#1583)

Documentation/versioning.md

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+# Versioning and Releases
+
+Note: This document references terminology defined at http://semver.org.
+
+## Release Frequency
+
+Regular MINOR releases of gRPC-Go are performed every six weeks.  Patch releases
+to the previous two MINOR releases may be performed on demand or if serious
+security problems are discovered.
+
+## Versioning Policy
+
+The gRPC-Go versioning policy follows the Semantic Versioning 2.0.0
+specification, with the following exceptions:
+
+- A MINOR version will not _necessarily_ add new functionality.
+
+- MINOR releases will not break backward compatibility, except in the following
+circumstances:
+
+  - An API was marked as EXPERIMENTAL upon its introduction.
+  - An API was marked as DEPRECATED in the initial MAJOR release.
+  - An API is inherently flawed and cannot provide correct or secure behavior.
+
+  In these cases, APIs MAY be changed or removed without a MAJOR release.
+Otherwise, backward compatibility will be preserved by MINOR releases.
+
+  For an API marked as DEPRECATED, an alternative will be available (if
+appropriate) for at least three months prior to its removal.
+
+## Release History
+
+Please see our release history on GitHub:
+https://github.com/grpc/grpc-go/releases