Document release process

RELEASE.md

+# Gitaly-proto release process
+
+## Requirements
+
+- Ruby
+- Bundler
+- Go 1.8
+
+## 1. Install dependencies
+
+If you have done a release before this may not be needed.
+
+```
+_support/install-protoc
+```
+
+## 2. Release
+
+This will:
+
+- do a final consistency check
+- create a version bump commit
+- create a tag
+- build the gem
+- ask for confirmation
+- push the gem and the tag out to the world
+
+```
+_support/release 1.2.3
+```