Adding a recipe for el2markdown.

There are other packages to convert elisp headers to markdown, I'm aware
of:

https://github.com/mgalgs/make-readme-markdown
https://github.com/thomas11/md-readme

I prefer this one, as it handles `;;; Foo:` (treating it as a header)
and Emacs style `foo' quoting correctly.
Merge pull request #2126 from Wilfred/el2markdown

Adding a recipe for el2markdown.