doc: Mention Span in developer-notes.md
doc: Mention Span in developer-notes.md

Former-commit-id: c987a9f4b9ce90bc6b45b61e4dd81e8e3c89d642
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin