Update ReadMe for HTTP Callbacks

README.md

@@ -40,6 +40,22 @@ See [LevelDB Persistence](https://github.com/yjs/y-leveldb) for more info.
 PORT=1234 YPERSISTENCE=./dbDir node ./node_modules/y-websocket/bin/server.js
 ```
 
+**Websocket Server with HTTP callback**
+
+Send a debounced callback to an HTTP server (POST) on document update.
+
+Can take the following ENV variables:
+- CALLBACK_URL : Callback server URL. 
+- CALLBACK_DEBOUNCE_WAIT : Debounce time between callbacks. Defaults to 2 seconds.
+- CALLBACK_DEBOUNCE_MAXWAIT : Maximum time to wait before callback. Defaults to 10 seconds.
+- CALLBACK_TIMEOUT : Timeout for the HTTP call. Defaults to 5 seconds.
+- CALLBACK_OBJECTS : JSON of shared objects to get data. ('{"SHARED_OBJECT_NAME":"SHARED_OBJECT_TYPE}')
+
+```sh
+CALLBACK_URL=http://localhost:3000/ CALLBACK_OBJECTS='{"prosemirror":"XmlFragment"}' node ./node_modules/y-websocket/bin/server.js
+```
+This would send a callback to localhost:3000 every 2 seconds (default DEBOUNCE_WAIT) with the data of an XmlFragment named Prosemirror in the body.
+
 ### Scaling
 
 These are mere suggestions how you could scale your server environment.