Power up docs

Improves the intro in the README.  Now tells the user which version of Puppet and Emacs we support, and [what Puppet actually is (for folks who can't google)](http://www.reddit.com/r/emacs/comments/20bnj3/puppet_mode_03_release_new_commands_and_major/cg23b9w).  I've also added all features to the commentary of the mode.

Last, but not least, I've referenced Flycheck in the docs for on-the-fly linting.  I think it's a useful addition to Puppet Mode, but I'm unsure whether that's ok.  After all, it's my own project, too, so it smells like nasty advertising, doesn't it?  I'd like to hear your opinion, @bbatsov