readme: Fix rendering of Markdown syntax

* Rename to .dm so that gitblit and github render it as
  Markdown (I'm deriving from the "-----" line that this is
  intended as Markdown).
* Use HTTPS for mediawiki.org.
* Wrap code in backticks.

Change-Id: I3951d9ac74a9c595a284426a8f79274071b57bda