Add documentation to discipline

[yilinwei]

Discipline is awesome. It's documentation is not.

Opening an issue which may or may not be started at hack the tower day to update the documentation so there's a quickstart for new users.

[changlinli]

Could we just basically copy over this blog post into an sbt-microsite (like a lot of the Typelevel ecosystem) as a starting point? If a maintainer is willing to take a look, I'm willing to make the pull request.

[chrisphelps]

I would be interested in helping on this, and @changlinli's suggestion seems a good starting point to me.