Add documentation for updown

README.md

@@ -36,6 +36,7 @@ When Newt receives WireGuard control messages, it will use the information encod
 - `secret`: A unique secret (not shared and kept private) used to authenticate the client ID with the websocket in order to receive commands. 
 - `dns`: DNS server to use to resolve the endpoint
 - `log-level` (optional): The log level to use. Default: INFO
+- `updown` (optional): A script to be called when targets are added or removed.
  
 Example:
 
@@ -92,6 +93,18 @@ WantedBy=multi-user.target
 
 Make sure to `mv ./newt /usr/local/bin/newt`!
 
+### Updown
+
+You can pass in a updown script for Newt to call when it is adding or removing a target:
+
+`--updown "python3 test.py"`
+
+It will get called with args when a target is added: 
+`python3 test.py add tcp localhost:8556`
+`python3 test.py remove tcp localhost:8556`
+
+Returning a string from the script in the format of a target (`ip:dst` so `10.0.0.1:8080`) it will override the target and use this value instead to proxy.
+
 ## Build
 
 ### Container