Added docpad to the tools listing as it supports CoffeeKup

[balupton]

DocPad is a static site generator and CMS which supports CoffeeKup. Personally, CoffeeKup is my favourite markup to use it with :-)

Wrote this pull request as perhaps it'll help other people get started with coffeekup too :-)

[RobLoach]

@balupton Looks like CoffeeKup is rather unmaintained. Last commit was two years ago, and there have been pull requests open for the past three years.

[RobLoach]

Actually, it's over at https://github.com/gradus/coffeecup now.