Clarify verifying double documentation.

* Moves introduction to README so it shows on the index page.
* Made intro text more generic to account for object doubles.
* Rename old introduction feature to "instance doubles" for symmetry
  with class and object double features.
Copy changes and extra link, addressing feedback.
Flesh out instance double docs.
Merge pull request #460 from rspec/issue-456

Clarify verifying double documentation.