WIP Modules FAQ/README/etc#

This is a WIP file in which we capture aspects of the modules implementation that need documenting. Some take the form of FAQ-like questions, others are simply details which we need to ensure are documented. Some of the points listed below could be part of a modules reference doc, others a modules FAQ... We don't seek to make a decision on what docs/FAQs etc should exist, just use this opportunity to capture the bits that need coverage.

Some of the questions below are presented with answers, many are just left here as a TODO.

The goal of this doc is to capture things in a central VCS-based file, alongside the code that "raises" the question/similar.

• Documentation around defaulting the value of CUE_REGISTRY (or whatever we call it) to the central registry.
• Document that pkg, usr and gen directories cannot co-exist with modules, and make clear what users (especially of gen) should probably do instead.
• Document potential future vendor behaviour.
• Document what we mean by a "canonical" version, how this differs from a "well-formed" version. Present examples of each.