Update README.md

Bug #93 : Clearer link to the main reference document with an overview of the main features of Mockito (organized by use case rather than by release date).
It's on the same line because they do the same thing. If it changes I'd update the documentation and provide a release note. Release note are just for that, how to keep up with changes. This README should contain an overview of the best recommended usage and features mostly for new users.