Improve readme for vimmers

Many (most?) vim users install plugins through vundle and it's a little not obvious how to do it because merlin is not just a vimscript plugin. I've added instructions to clarify that. Also, neocomplcache is a really popular completion plugin so I've added a small vimscript snippet to make it work with merlin. Idea being not having to press C-x C-o anytime you'd like merlin to complete for you.   

Finally, I want to say that merlin is awesome from what I'm seeing :D keep it up!