added release instructions

RELEASE.md

@@ -0,0 +1,14 @@
+Releasing Toolbelt v4
+=====================
+
+Prerequisites:
+
+* `gem install rake aws-sdk`
+* `heroku/heroku-cli` repo
+* `HEROKU_RELEASE_ACCESS` and `HEROKU_RELEASE_SECRET` (from Toolbelt v3)
+
+Increment [version](https://github.com/heroku/heroku-cli/blob/master/version).
+Update [CHANGELOG](https://github.com/heroku/heroku-cli/blob/master/CHANGELOG).
+[optional] Update dev branch with latest changes.
+[optional] Run `rake release` on dev branch to test changes.
+Run `rake release` on master branch.