Add rules for partitioning multiple repositories

[jmcgeheeiv]

I recently had a lot of experience with multirepo, so I added my rules and recommendations here.

[paul-hammant]

Thanks John, but I've gone with different wording to what you contributed - https://github.com/paul-hammant/tbd/commit/491810b2eae69741d8b11d6aba25ab0b1201def2