warn about config errors

howto/nagios.md

@@ -17,9 +17,18 @@ Nagios/Icinga service for Tor Project infrastructure
     : NCMD is the command that's being run. If it looks sane, run it. With --verbose if you like more output.
     ssh -t TARGET.tpo "$NCMD" --verbose
 
-## How to add a host
+## Changing the Nagios configuration
 
-- Add new hosts to `tor-nagios/config/nagios-master.cfg`.
+Hosts and services are managed in the `config/nagios-master.cfg` YAML
+configuration file, kept in the `admin/tor-nagios.git`
+repository. Make changes with a normal text editor, commit and push:
+
+    $EDITOR config/nagios-master.cfg
+    git commit -a
+    git push
+
+Carefully watch the output of the `git push` command! If there is an
+error, your changes won't show up (and the commit is still accepted).
 
 ## Forcing a rebuild of the configuration