Add some keywords in README file

I spent a few hours to understand how to prevent Paloma from adding callback code after the view, and my Google searches were unsuccessful with keywords like "rails paloma no js in view", or "rails paloma ajax response".

I eventually found how to do it thanks to the changelog and a grep command on the gem code, but I think adding these keywords in the README would help other users to find about not displaying the callback in the view. 

Thank you for your work anyway, this gem is great.