Reformatted README as Markdown, with slight reformatting.

Markup other than source (of whatever format), rendered as text, always bugs me. So I took a few minutes to reformat the README as Github-Flavoured Markdown, including

* section titles reformatted as headers;
* a table of contents;
* project links list now has descriptive names rather than raw URLs;
* URLs embedded in or referred to in text now have hyperlinks;
* code formatted in code blocks.