Convert README to Markdown, clean up.

The Markdown format is convenient for viewing on Github.
Add html target to generate HTML using coqdoc
Replace README wih README.md
Add a link to Github in README.md
Add instructions for compilation of coqdoc
Spellcheck the other README.
Further small improvements to README.md
Link README.md to README.txt
Merge pull request #24 from andrejbauer/vv-master-for-coq8.4pl3patched

Vv master for coq8.4pl3patched