Readme overhaul, moved bits into Wiki

There are a few pages which go along with it, still work in progress but early feedback/help is welcome:

https://github.com/Particular/GitVersion/wiki/Choosing-branching-strategy
https://github.com/Particular/GitVersion/wiki/FAQ
https://github.com/Particular/GitVersion/wiki/GitFlow
https://github.com/Particular/GitVersion/wiki/GitFlowExamples
https://github.com/Particular/GitVersion/wiki/GitHubFlow
https://github.com/Particular/GitVersion/wiki/GitHubFlowExamples