update README with _FILE env vars (#91)

* update README with _FILE env vars

* Update README.md

* Update README.md

---------

Co-authored-by: fkwp <fkwp@users.noreply.github.com>

Author: gaelgatelement

README.md

@@ -52,8 +52,8 @@ The service is configured via environment variables:
 Variable | Description | Required
 --- | --- | ---
 `LIVEKIT_URL` | The websocket URL of the LiveKit SFU | Yes
-`LIVEKIT_KEY` | The API key for the LiveKit SFU | Yes
-`LIVEKIT_SECRET` | The secret for the LiveKit SFU | Yes
+`LIVEKIT_KEY` or `LIVEKIT_KEY_FILE` | The API key or key file path for the LiveKit SFU | Yes
+`LIVEKIT_SECRET` or `LIVEKIT_SECRET_FILE` | The secret or secret file path for the LiveKit SFU | Yes
 `LIVEKIT_JWT_PORT` | The port the service listens on | No - defaults to 8080
 
 ## Disable TLS verification