Add README and others into gemspec

I noticed that the README.md file was missing from the auto-generated docs at http://rubydoc.info/gems/chefspec/frames - I'm pretty sure this is what's necessary to correct this. rubydoc.info lands people on the README if it's present, and other files are available from the files tab in that link.

I also figured that the license and changelog deserved to be packaged along with the distributed gem