When I run with "-i", I'm noticing lots of lines like this:

Task ':runFeatures' has not declared any outputs.

I've got a fair number of tasks that perform expensive work to generate files (e.g. install Ruby gems, fire off a JavaCC process), so if I could prevent those tasks from executing based on the existence of certain files, that'd be awesome.

However, I don't see anywhere in the User Guide that talks about how to declare outputs for tasks. Am I missing it? If not, we should add something.