The stacks.md document used to describe the metadata format. Now that we've
moved away from the old model of YAML + Go templates, and created the MuPack/MuIL
format, this document needed to be overhauled.
It's pretty bare bones now, however it will eventually evolve into a document
describing how the Mu abstractions map down onto MuPack/MuIL concepts. For example,
it describes how subclassing mu.Stack creates a "stack", even though at the MuPack/MuIL
level it's just any old subclass.
