Include docs and LICENSE in the gem

Hi, I just started looking at packaging foodcritic [for Debian](http://bugs.debian.org/711220) and the first order of business is to make sure that the licensing is sane.

Can we please include LICENSE in the gem. Right now, there is only a reference to the MIT license in the `metadata.yml`, but no actual license statement...

Other documentation is also a nice-to-have. If there are unit-tests that can be run at package build time, we should probably also include those, but I haven't looked into that yet.

I'm fairly new to Ruby and have no experience in packaging it, so hope this is all sane...