Document the release workflow

[kgiusti]

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.