Serf reporter: add note on potentially stale configuration files.

README.md

@@ -94,6 +94,7 @@ You can use Nerve together with [Serf](https://www.serf.io/), which requires:
 * `serf_config_dir`: a directory to place files in that is readable/writable by both Serf and Nerve. 
     * Nerve will place configuration files named `zzz_nerve_<some-service>_<port>.json` under the directory specified with `serf_config_dir`. 
     * Serf will read these configurations and must be configured to run with `-config-dir=<serf_config_dir>`
+    * Please note that it is up to you (through e.g. configuration management code) to ensure no invalid configuration files are left under `serf_config_dir` by wiping all `zzz_nerve*` files when renaming/removing services. (each nerve reporter creates/deletes files for services named `<service-name>`, but renaming `<service-name>`  to `<service-name2>` on an existing machine could lead to stale configuration files remaining under `serf_config_dir`).
 * Nerve must have permissions to reload the serf process. (can be specified by the `serf_reload_command` parameter - defaults to `/usr/bin/killall -HUP serf`)
     * (You can for example install Serf and Nerve to run under the same user)