Add an after_bundle callback in Rails templates

The template runs before the generation of binstubs – this does not
allow to write one, that makes an initial commit to version control.
It is solvable by adding an after_bundle callback.
Add after_bundle to the release notes and upgrade guide
Merge pull request #16359 from skanev/after-bundle-in-rails-templates

Add an after_bundle callback in Rails templates

Conflicts:
	railties/CHANGELOG.md