I think I would expand a little more on the second sentence "...make your changes." e.g.

> To contribute, fork the Subliminal repo, clone your fork, then make your changes on a branch. GitHub has [a guide](https://help.github.com/articles/fork-a-repo) to this process if you'd like more information.

Then, I'd make "If you're adding functionality..." the start of a new paragraph. We might also finish that paragraph like

> ...no new tests. Subliminal tests fall into two types: