Update README to explain Heroku support for persistent schema's.

Since Rails 4.1 the database.yml isn't completely overwritten which
enables an easy solution for persistent schema's on Heroku. See Heroku
changelog item: https://devcenter.heroku.com/changelog-items/426
Merge pull request #140 from stefanroex/update_readme_heroku

Update README to explain Heroku support for persistent schema's.