zulip_botserver: Document bot-config-file option.

Author: mpagler

zulip_botserver/README.md

@@ -19,6 +19,7 @@ The format for a configuration file is:
     [email protected]
     site=http://localhost
     token=abcd1234
+    bot-config-file=helloworld.conf
 
 Is passed `--use-env-vars` instead of `--config-file`, the
 configuration can instead be provided via the `ZULIP_BOTSERVER_CONFIG`