Add RPC deprecation process

DEPRECATION.md

+# RPC deprecation process for gitaly-proto
+
+First create a deprecation issue at
+https://gitlab.com/gitlab-org/gitaly/issues with the title `Deprecate
+RPC FooBar`. Use label `Deprecation`. Below is a template for the
+issue description.
+
+```
+We are deprecating RPC FooBar because **REASONS**.
+
+- [ ] put a deprecation comment `// DEPRECATED: <ISSUE-LINK>` in gitaly-proto **Merge Request LINK**
+- [ ] find all client-side uses of RPC and list below
+- [ ] update all client-side uses to no longer use RPC **ADD Merge Request LINKS**
+- [ ] wait for a GitLab release in which the RPC is no longer occuring in client side code **LINK TO GITLAB-CE RELEASE TAG**
+- [ ] delete the server side implementation of the old RPC in https://gitlab.com/gitlab-org/gitaly **Merge Request LINK**
+```

README.md

@@ -82,6 +82,10 @@ gRPC provides an implementation framework based on these Protobuf concepts.
     clients.
 1.  Each RPC `FooBar` has its own `FooBarRequest` and `FooBarResponse`
     message types.
+1.  We never make backwards incompatible changes to an RPC that is
+    already implemented on either the client side or server side.
+    Instead we just create a new RPC call and start a deprecation
+    procedure (see below) for the old one.
 ## Contributing
@@ -92,3 +96,7 @@ for changes. Some of the code in the Go client libraries is sensitive
 to implementation details of the Go standard library (specifically,
 the ouput of gzip). **Use the same Go version as .gitlab-ci.yml (Go
 1.8)** when generating new client libraries for a merge request.
+
+## How to deprecate an RPC call
+
+See [DEPRECATION.md](DEPRECATION.md).