I've just been reading around about the sort of content we can use on Gist, Github issue tracker issues and comments, and it turns out there's an awesome, slightly unusual feature that Github markdown supports.
If you want a to-do list, then all you need to do is start each item in the list with:
- [ ] Something I need to do.
This is rendered as:
- Something I need to do.
Ticking and un-ticking this box updates the underlying text in the markdown. Awesome!