Add webhook_secret sample to proxy example

README.md

@@ -54,6 +54,7 @@ webhook = True
 webhook_host = 127.0.0.1
 webhook_port = 3333
 external_url = http://bad.code.brought.to.you.by.maxpowa.us/webhook
+webhook_secret = a_random_secure_string_here_!!123@
 ```
 The above configuration is only listening on `localhost (127.0.0.1)`, because I'm using a reverse proxy in nginx to proxy `/webhook` to port 3333. The reverse proxy configuration would be fairly simple, as shown below. Auth must be included, to match the "Authorization callback URL" you included in generating the API keys.
 ```