Add git config instructions for Ubuntu and Windows

[allait]

Copied the "Configuring Git" from OS X setup instructions to Ubuntu and Windows (changing "terminal" to "command prompt").

Deploying to Heroku requires making a git commit, and git commit will complain if user.name and user.email are not set. And since there are instructions to set up ssh keys it probably makes sense to do this at this stage.

[davidchambers]

LGTM

[bridgethillyer]

+1 I think this is a good set of instructions. I bet we'll iterate through this one some more as people's different environments make this on a particularly "choose your own adventure" type of setup.

Thanks @allait!