Added documentation on available colors; fixes #26

I added a documentation block to explain the colors available in ack. This information was pulled directly from the [Stack Overflow answer](http://stackoverflow.com/questions/9625316/what-color-options-exist-for-ack-grep-for-colorization-of-output-logs-etc/9626444#9626444) that detailed the available options.

If this is the wrong place to put it, let me know.