document spec change doc format and process

Before we define this as "1.0", we need to document how to make changes to the spec and what process we use to take a change from idea to proposal to acceptance.

We've been roughly following a sketch of a process that I put together before we had an opportunity to try following it, and unsurprisingly, we've been finding places where it needs to change before being finalized here.

Points we need to hit include:

- [ ] Templates for changes
- [ ] Templates for PRs
- [ ] Templates for issues
- [ ] Repo organization (where to changes go, how to they reflect on spec docs, etc)
- [ ] Process for accepting a change into the spec
- [ ] Process for accepting a proposal that needs more review before integrating into the spec