update readme

README.md

@@ -93,6 +93,8 @@ The `--network=host` flag is mandatory since Docker can change the source port o
 docker run --rm -it -e RTSP_PROTOCOLS=tcp -p 8554:8554 -p 1935:1935 aler9/rtsp-simple-server
 ```
 
+Please keep in mind that the Docker image doesn't include _FFmpeg_. if you need to use _FFmpeg_ for a custom command or anything else, you need to build a Docker image that contains both _rtsp-simple-server_ and _FFmpeg_, by following instructions [here](https://github.com/aler9/rtsp-simple-server/issues/183#issuecomment-760856015).
+
 ## Basic usage
 
 1. Publish a stream. For instance, you can publish a video/audio file with _FFmpeg_: