Simplify introductory documentation.

I'm sending this PR as the outcome of today's first look at this project. As a newcomer, I'd like to see what's the _minimum_ amount of work needed to integrate the library into the project. 

Specifically, I had a moment of doubt in which I wondered if I really needed `puppetlabs_spec_helper` in order to use `rspec-system-puppet`. Also, I found the `.nodesets.yml` example quite long and confusing, since it turns out you only need one nodeset to run the specs (i left the second for consistency of a later paragraph). Also, the rspec-system README has a good paragraph describing how to manage nodesets.

Please regard this as a personal preference, so feel free to discard the PR if you don't agree with the expressed view.