Move SSH Key lesson from Git to Extras

Related with #691.
On Wed, Sep 10, 2014 at 09:24:04AM -0700, John Blischak wrote:
> Unfortunately this option also requires you to type your username
> and password every time you interact with GitHub.

Not anymore.  With credentials, you can cache your password for HTTPS
requests [1,2].

[1]: http://git-scm.com/docs/gitcredentials.html
[2]: https://help.github.com/articles/caching-your-github-password-in-git