Clarify readme and contributing

Greg asked volunteers to review these two files ([Discuss] volunteer wanted to clarify README.md and CONTRIBUTING.md) and I did. Feel free to comment.
On Thu, Jun 12, 2014 at 01:33:39PM -0700, r-gaia-cs wrote:
> Normally I will use http://jekyllrb.com/ but I agree that Wikipedia
> is more suitable in this case.

Alternatively, http://jekyllrb.com/docs/home/ seems to have a
reasonable “what is Jekyll” section.