Move usage bits up, clarify them a bit.

So, [apparently either the README isn't clear enough or people start losing attention after reading the first part of it](http://www.reddit.com/r/emacs/comments/21oo98/quelpa_is_emacs_crack_ie_why_was_this_not_posted/cgf3g5z). I've moved up the descriptive part from the usage to the top and reworded it a bit. Lets see whether this helps.