Update README

Trying to update the readme. I’d be glad for feedback, especially on the rewritten section dealing with the various options of running RSpec from the bundle. 

@elia, you’ve also dealed with that in https://github.com/elia/rspec.tmbundle/commit/e3f79627b415ee8b24fbb8befa8b8aa899569ab6 – could you elaborate why you went for forcing system Ruby 1.8? Were there any specific issues for you, and if so, are they still present with the current version of the bundle? Does the rewritten readme help solving these kinds of issues?