README.md: Document the syntax used in the README

README.md: Document the syntax used in the README

Explain that the file is written in Markdown, and link to the Markdown
spec and GitHub's markup parsing docs.

After adjusting README.md in my text editor, I compared my working
directory with the index using:

  $ git diff

That looked reasonable, so I committed all the modifications using:

  $ git commit --all

The `--all` option tells Git to automatically stage any modifications
or deletions.  I could have used the following:

  $ git add README.md
  $ git commit

and achieved the same result.  Some people like to stage changes
explicitly, and some like to stage them automatically.  Pick whichever
works best for you.