Register Recipient Registry method in Subgraph Contract

[daodesigner]

As an admin I would like to track the Recipient registry in the subgraph so that I may add people to the registry before a funding round.

• contract method
• contract event
• subgraph mapping

[pettinarip]

@daodesigner just checking the status of this issue. Let me know if we can close or move it to the monorepo as we are cleaning the issues in this repo.