[STORY] Great Documentation

[jonpugh]

As a Terra user I want great documentation So that I can use and extend Terra quickly and easily.

• [x] Create some great markdown files for README.md and install.md.
• [x] Set up terra.readthedocs.org

[jonpugh]

This is moving so I'm going to close it.

If we ever stop documenting as we build, let's re-open this story.