Add release doc

Author: seanabrahams

RELEASE.md

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+# Release process
+
+This document simply outlines the release process:
+
+1. Ensure you are running on the oldest supported Elixir version (check `.travis.yml`)
+
+2. Ensure `CHANGELOG.md` is updated and add current date
+
+3. Change the version number in `mix.exs` and `README.md`
+
+4. Run `mix test` to ensure all tests pass
+
+5. Commit changes above with title "Release vVERSION" and push to GitHub
+
+    git add .
+    git commit -m"Release vX.Y.Z"
+    git push origin master
+
+6. Check CI is green
+
+7. Create a release on GitHub and add the CHANGELOG from step #2 (https://github.com/graphql-elixir/graphql/releases/new) using VERSION as the tag and title
+
+8. Publish new hex release with `mix hex.publish`
+
+9. Publish hex docs with `mix hex.docs`
+
+## Deprecation policy
+
+GraphQL deprecations happen in 3 steps:
+
+  1. The feature is soft-deprecated. It means both CHANGELOG and documentation must list the feature as deprecated but no warning is effectively emitted by running the code. There is no requirement to soft-deprecate a feature.
+
+  2. The feature is effectively deprecated by emitting warnings on usage. In order to deprecate a feature, the proposed alternative MUST exist for AT LEAST two versions.
+
+  3. The feature is removed. This can only happen on major releases.