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).
That's not true for `BDDMockito.then` :wink: 
I think you probably meant this link instead : [`then`](http://mockito.github.io/mockito/docs/current/org/mockito/stubbing/OngoingStubbing.html#then(org.mockito.stubbing.Answer))