Document how Sponsor codes work

Wiki page How we do project management at The Tor Project mentions the concept of a "Sponsor", but it could also document the sponsor code logic, including:

1. Whether they're tied to a project, a contract, an actual sponsor etc.
2. How they're assigned.
3. How (often) they can change.
4. How to handle sponsor codes in the documentation/code/repositories.
5. Other useful information.