Capture our heuristics for configuration.

Right now it's only hinted at in incubator + a single email. Lets have
a place to point folk at to read about it. Alternatively we could put
it in incubator, but I think the image-elements repository is shaping
up to be the prime repository of code for us.

Change-Id: Ief5a33887239d313a76dfca5cd78c6f9263873bf
Merge "Capture our heuristics for configuration."