Add protocol sequence diagram for dual funding
Diagram is found at https://gitlab.com/lightning-signer/docs/-/blob/master/seq-diagrams/README.md
Other protocol sequence diagram issues: #5 (closed), #6 (closed), #7
Diagram is found at https://gitlab.com/lightning-signer/docs/-/blob/master/seq-diagrams/README.md
Other protocol sequence diagram issues: #5 (closed), #6 (closed), #7