<feature> Add RELEASE_CHECKLIST.md template

.github/RELEASE_CHECKLIST.md

@@ -0,0 +1,20 @@
+How to release a new AdminThemeBundle version
+======================================
+
+1. Create the tag and sign it. Example: `git tag -s v1.23.4` (the tag version is
+   always prefixed with `v`, but remove it for the tag comment: `1.23.4`).
+5. Push the tag to GitHub. Example: `git push origin v1.23.4`
+6. Prepare the changelog of the new version with the custom `changelog` Git
+   command. Example: `git changelog v1.23.4` (the version passed to the command
+   is the previous version used as a reference to list the changes).
+7. Go to https://github.com/avanzu/AdminThemeBundle/releases and click
+   on `Draft a new release`. Select the tag pushed before and paste the
+   changelog contents.
+
+Resources
+---------
+
+The custom `changelog` Git command used to generate the version changelog can
+be defined as a global Git alias:
+
+    $ git config --global alias.changelog "!f() { git log $1...$2 --pretty=format:'[%h] %s' --reverse; } ; f"