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

Former-commit-id: 53fc7824d62084f705221b0c1c8f45320dfc1805
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin