Document version numbering schema (i.e. semver.org or otherwise)

[d33bs]

We currently have followed a loosely semantic versioning scheme in alignment with PEP 440. While this is generally followed it's not yet documented explicitly anywhere. We should update the docs to help reflect this so as to navigate ambiguities when it comes to version numbering.

Originally brought up from a question @gwaybio raised in #498 .