Edits the README to clarify a few methods.

Hey,
Leaflet-rails is a great gem, but I got a little confused on how to implement the tiles with the ```#map``` method.  I even ran across an unanswered [stackoverflow post](http://stackoverflow.com/questions/22900904/setting-up-leaflet-with-ruby-on-rails/23644986#23644986). So I edited the README just to clarify things a bit, wish I could help more.