feature: the firewall lets the Router Manager in on the internal interface.

README.md

@@ -502,6 +502,14 @@ request reads the token secret from `vpn-router.json`, and the features and the
 before that file exists the page shows no features. `vpn-router-setup` opens `web.port` on the
 internal interface in the firewall.
 
+## Router Manager
+
+The `vpn-router-manager` package, built in the repository `linux-cloud-router-manager`, is a web
+application that shows the state of the router and manages its users through the status web
+service. It can be installed on the router or on another Linux system. While it is installed on
+the router, `vpn-router-setup` opens its port (`port` in `/etc/vpn-router-manager/manager.json`,
+8090 by default) on the internal interface; restart `vpn-router-setup` after changing it.
+
 ## Removal
 
 `apt-get remove` stops `vpn-router-docker`, which removes the Docker rules. It terminates the