add warning about app/assets/

Because of https://github.com/sstephenson/sprockets/issues/347, Rails will complain if it tries to compile files without extensions. https://github.com/rails/rails/pull/7968 made it so `app/assets/` is the only directory that gets precompiled by default, so added a warning to not use this.

FYI, I'm now recommending this gem in [my Bower+Heroku guide](https://gist.github.com/afeld/5704079/). Nice work!