Add a note about CocoaPods to the README

Add a small note about installing hoedown via CocoaPods.

I created the hoedown spec, because I use CocoaPods extensively in my projects and in the past weeks I really fell in love with hoedown, so I had to add it to CocoaPods/Specs ;)

See: https://github.com/CocoaPods/Specs/blob/master/hoedown/2.0.0/hoedown.podspec