Improvements in Readme

[LucasCioffi]

Hello, I love the gem. The README file would be a little stronger if it specified that there needs to be a migration which adds a latitude and a longitude attribute on the users table. It would also be helpful to make it clearer that the following section in the README is supposed to replace some of the code in step 5:

markers = handler.addMarkers(<%=raw @hash.to_json %>);

Thanks again for an awesome gem!

[apneadiving]

Please submit a pull request with the changes :)