Explain how to contribute to gitflow WITH gitflow and GitHub

[dolmen]

The README should explain how to initialise a working environment to contribute to gitflow.
The only instruction in the README is currently:
git clone --recursive git://github.com/nvie/gitflow.git

However:

• this does not download all the branches used for the gitflow workflow
• this does not tell to use "git flow init"
• "git flow init" wants to create missing branches instead of using the existing ones because they have not been downloaded.

[nvie]

Closed by #170.