Post on Lesson Metadata

Draft of blog post on metadata and lesson templates, not ready for merge
These are nitpicks, sorry:

- no `www.` before GitHub URLs
- neither of these are valid URLs for a GitHub repo. They could be `github.com/user/repo/tree/commitid` and `github.com/anotheruser/anotherrepo`, respectively. With things like `pip` the commit is often identified with an `@`.