Add org-mode syntax doumentation

[JonathanReeve]

There's plenty of markdown syntax documentation in the manual, but no org-mode syntax documentation, that I could find. This would be useful to org-mode users.

[lierdakil]

Honestly, I don't use org-mode and have no idea whether it works at all. Feel free to contribute the documentation you want however, hopefully I'll be able to extract tests from that to ensure I don't accidentally break something (or you could also contribute tests if that tickles your fancy).