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.

I see that md-readme is already present on MELPA, does this conflict with MELPA's principle of being a curated collection of packages?