Add mention of rtrmon

Author: lspgn

README.md

@@ -10,6 +10,7 @@ GoRTR is an open-source implementation of RPKI to Router protocol (RFC 6810) usi
 * `/prefixfile` contains the structure of a JSON export file and signing capabilities.
 * `/cmd/gortr/gortr.go` is a simple implementation that fetches a list and offers it to a router.
 * `/cmd/rtrdump/rtrdump.go` allows copying the PDUs sent by a RTR server as a JSON file.
+* `/cmd/rtrmon/rtrmon.go` compare and monitor two RTR servers (using RTR and/or JSON), outputs diff and Prometheus metrics.
 
 ## Disclaimer