Document the release workflow

This project has automated the release process.

Document the release process workflow, including:

- what triggers the release?
- what is the versioning semantics (e.g. what causes a "fix release" vs a "feature release"?)
- is there a manual method for creating a release? If so document it.
- what are the outputs of the release process? (e.g. platform-specific packages? tar files? etc).

Acceptance criteria: a RELEASING.md document is added to the repo with the above content.