Add info on using a .gitkeep to push migrations directory

As noted by @katbow in https://github.com/dwyl/library/pull/4#pullrequestreview-69195376 it's really useful to use a [.gitkeep](http://www.ryanwright.me/cookbook/git/gitkeep) so that git tracks your empty migrations directory (when you've just created a phoenix project and haven't made any migrations yet). It also means that your travis build will still pass. Without a migrations directory the build will fail.

We should add this info to the readme!

Start in the README and review the guidance around creating a Phoenix project and its migrations directory. Document that adding a .gitkeep keeps an otherwise empty migrations directory in Git and allows the Travis build to pass; done when the README explains this setup for projects without migrations.

Written by the indexing model from the issue text.