DOC: regenerate CONTRIBUTING.md
This is an alternative to #9796.

I simply did a couple of changes to the restructuredtext source so it generates cleaner markdown. The main thing was making the categories not quite so nested. (Was that [heavy nesting](http://pandas-docs.github.io/pandas-docs-travis/contributing.html#committing-your-code) intentional?) I think pandoc was running out of header styles or something like that.

cc @rockg @jreback @jorisvandenbossche
