Reword the readme so that it's not so intimidating

I was very excited by the concept of the a11yproject, but was immediately scared off by the homepage readme that said "in order to contribute, you need to know something about Jekyll, Compass, monkeys, eastern european swallows .... " etc. 

It took a little poking around to figure out that those requirements were for contributing to the *code* of the site, not contributing *content*.

I'm just suggesting updating the readme so it's a little more clear in that regard.