Describe library in README.md
Describe library in README.md

The previous README was only a scaffold.
The current README describes the matches made available by this library, following the structure of [rspec-expectation's README](https://github.com/rspec/rspec-expectations/blob/master/README.md)

groyoh/rspec-collection_matchers,rspec/rspec-collection_matchers,rspec/rspec-collection_matchers,groyoh/rspec-collection_matchers