Amended README with note to clear local sprockets cache after modifying ...

Changes to `templates_root` don't take effect until `/tmp/cache/assets` is cleared. This came up on [a stack overflow question](http://stackoverflow.com/questions/14948410/how-can-i-specify-an-alternative-directory-for-my-handlebarsjs-templates-with-th) and can be quite mystifying. I think it merits mention in the README.