Mention the "after_commit" callback in the README

[donv]

So that new developers can know that it exists, and know when it is called.

[rafaelfranca]

Feel free to submit a pull request.

[donv]

Hi @rafaelfranca !

Thanks for the quick response. I'll submit a PR, but I have one question first: Should "after_commit" be called for an update that is stopped by the "dirty" module since there were no changes? I would expect not, but I think I see this in at least one of our apps.

Just to clarify for the README change.

[rafaelfranca]

If no commits are made to the database it should not run. If it is happen seems like a bug.