Add link to ZIP212 in coinbase comment

Co-authored-by: Daira Hopwood <daira@jacaranda.org>

src/main.cpp

@@ -970,6 +970,7 @@ bool ContextualCheckTransaction(
                 // decrypted to a note plaintext, MUST have note plaintext lead byte equal
                 // to 0x02. This applies even during the grace period, and also applies to
                 // funding stream outputs sent to shielded payment addresses, if any.
+                // https://zips.z.cash/zip-0212#consensus-rule-change-for-coinbase-transactions
                 if (canopyActive != (encPlaintext->get_leadbyte() == 0x02)) {
                     return state.DoS(
                         DOS_LEVEL_BLOCK,