update README

Made 3 distinct steps, clarified packaging, moved running bot at end (not needed by most people), ensured correct capitations.

Diff is confusing; easier just to [read directly](https://github.com/tomByrer/libgrabber/blob/master/README.md)
[jsDeliver blog](http://blog.jsdelivr.com/2014/06/public-cdn-auto-updated.html) links to an issue that deep-links to https://github.com/jsdelivr/libgrabber#updatejson-schema
So I'm preserving the deep link