readme: Fix broken Markdown list syntax

Hash symbols create a level 1 heading:

* https://git.wikimedia.org/blob/oojs%2Fui.git/0a078dc25/README.md
* https://github.com/wikimedia/oojs-ui/tree/0a078dc25#readme

Change-Id: Ia95851833f94c6b098c15540be0e3cd83beb6bac