Add a document covering Django Evolution's versioning policy.

Review Request #11243 — Created Oct. 24, 2020 and submitted — Latest diff uploaded


Django Evolution


As of 2.0, Django Evolution uses semantic versioning. Prior releases
didn't have any versioning policy, so it's not obvious, and as such it's
booa easy to avoid thinking about proper deprecation and upgrade paths.
What parts of the project would impact the version also wasn't defined.

This change adds a Version Policy document that specifies the policy,
and when major/minor/micro versions will be updated. It also organizes
the codebase into Public API vs. Private API categories so it's clear
what changes will require version bumps.

Built the docs and read through it. Checked for errors.