Formatted blog post on extracting history

Includes content from #692 formatted as HTML and edited for style.
do you mean it doesn't have to be easy?

I'm thinking of users that just want to be able to pull a lesson off the shelf. Do you plan to have some index of the canonical lessons so that is easy to do? If swcarpentry has a collection of standard lessons, this is as easy as `git clone https://github.com/swcarpentry/novice-git`.