Proposal for how ownership transfer works

[taoeffect]

Problem

When an owner of a group deletes their identity contract it will delete the group.

Before doing this, we want to be able to transfer ownership to someone else.

Solution

Create a wiki page for a proposal for how this is done.

Also determine (as soon as possible) whether or not this will require recreating groups after https://github.com/okTurtles/group-income/milestone/3 "Final breaking changes" is deployed.