PSA: Extending BIP 70 / payment protocol

Please put any payment protocol extensions into a new draft BIP. BIP 70 should not be updated continuously.

Mimic the IETF, where new RFCs are produced as protocols are extended. Once published, an IETF RFC is considered static.

That said, I think it is OK to be smart, and break the "static" rule a bit for significant BIP bugs, or harmless maintenance of links-to-resources.