Document the package maintenance - ruby, docker, etc.

[Gredin67]

@Salamandar thanks a lot for solving the upgrade to v6 !
Could you explain a bit what the issues were and how to solve it ?
Ideally it would be nice to have some instructions in the package README to explain the specifics of this package.
Usually packaging for yunohost only requires to understand deploy instructions, but here, with ruby sh*** and deploy instructions based on docker, I don't understand how I could maintain the package if nobody else does..