Add -prometheusmetrics to release notes

doc/release-notes.md

@@ -4,3 +4,20 @@ release-notes at release time)
 Notable changes
 ===============
 
+Prometheus metrics
+------------------
+
+`zcashd` can now be configured to optionally expose an HTTP server that acts as
+a Prometheus scrape endpoint. The server will respond to `GET` requests on any
+request path.
+
+Note that HTTPS is not supported, and therefore connections to the endpoint are
+not encrypted or authenticated. Access to the endpoint should be assumed to
+compromise the privacy of node operations, by the provided metrics and/or by
+timing side channels. Enabling the endpoint is **strongly discouraged** if the
+node has a wallet holding live funds.
+
+To enable the endpoint, add `-prometheusmetrics=<host_name>:<port>` to your
+`zcashd` configuration (either in `zcash.conf` or on the command line). After
+restarting `zcashd` you can then test the endpoint by querying it with e.g.
+`curl http://<host_name>:<port>`.