rfds: Add signature RFD

[alexjg]

Here's a rendered version of this RFD.

Inspired by Oxide's RFD process I'm adding this document to discuss how we should sign commits in Beelay. The core problem is that Automerge has no concept of a commit Author at the moment, which means

a) We can receive multiple versions of the same commit from different peers
b) We need to figure out how to handle compressing signatures when we're compressing chunks of commits

Lot's more detail in the doc.