Update README files

* Rename README files to README.md and link from root.
* Rename HACKING to HACKING.md and link from root.
* Add additional links.
* A fair amount of added punctuation and rewording.
* Replace references to darcs/trac with git/github.
* Add title to HACKING.
* Ensure README.md is in .cabal files.
* Remove broken link to old wiki in HACKING.
* Reformat tests/README, add links to individual files.
* Add link to tests/README to HACKING.
* Add links to license files.