Add mention of Parker Moore (@parkr) in README.

By now, I have almost entirely rewritten Jekyll with the help of our awesome
contributors. Taking a look at the contributions graphs GitHub provides,
I have to date pushed 1,452 commits to master with 24,991 additions and
17,330 deletions. In contrast, Tom Preston-Werner has pushed 295 commits
and 13,461 additions and 6,806 deletions. I don't need to have my name all
over it, but I think I have worked hard enough over the last 21 months
to deserve to put my name alongside Tom's and Nick's.

The lovely thing about version control is that changes can always be reverted.

Thanks for launching this great product, @mojombo and @qrush. I hope I haven't
let you down.