Create guide for making rigs backwards compatible

[TheDuckCow]

As a continuation of #455, we want to make a clear guideline for how contributors can convert rigs from more recent versions of blender to older ones.

There's a set of known steps as well as limitations we could outline, and typically involves very specific versions of blender that are needed to go between versions (the Blender Foundation tends to be good about co-releasing backwards compatible code changes to the current LTS version).