Fix and clean up Markdown formatting in README

* Fix code blocks (2 space indent isn't enough for Markdown)
* Move reference links to end of file (to avoid interrupting reading flow)
* Add a few code spans and links
* Use sentences instead of bullet points where possible

For comparison:

* <https://github.com/luaposix/luaposix/blob/master/README.md>
* <https://github.com/craigbarnes/luaposix/blob/master/README.md>