docs: update contribution guidelines

Author: saecki

CONTRIBUTING.md

@@ -65,10 +65,11 @@ is replaced by the contents of the `docgen/shared/keymaps.lua` file.\
 The indentation of the placeholder has to be a multiple of 4 spaces
 and will be applied to the replacement file.
 
-## Releases
+## Release Checklist
 __Manual steps:__
-1. Update changelog and add migration guide
-2. Run github `Release` action and input new version
-2. Verify release
+1. Manage deprecations
+2. Update changelog and add migration guide
+3. Run github `Release` action and input new version
+4. Verify release
   - Publish draft-release
   - update `stable` tag