There is a template called Xcode-Plugin-Template. Install Alcatraz, install the template, and you're good to go :)

The template will set your new plugin up, so you'll be ready to start coding.

One IMPORTANT downside is that Xcode doesn't support ARC, so you'll want to write -retain, -release, -autorelease

Yeah, that was my question as well. I've often thought about plugins that I would want to write for Xcode, but there seems to be no documentation.