Update the README file for clarity regarding Javascript & CSS file usage

I ran into problems getting this to work (I missed crucial instructions in the "Style" section. I believe moving this information up nearer the front and calling it out as a basic configuration requirement would clarify the intent and make it easier for folks to understand what's required. I also ran into issues with Heroku and the asset pipeline and so added some information about that as well.

Let me know what you think; I'll be happy to make more changes if you think that would be helpful.