explain usage

README.md

@@ -10,6 +10,25 @@
 - syipy (`python3-scipy`) for interpolating the position of the cursor
 - ffmpeg (`ffmpeg`) for merging audio and video
 
+## How to use
+
+If your BBB recording URL looks like this 
+
+```text
+https://bbb.example/playback/presentation/2.0/playback.html?meetingId=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-aaaaaaaaaaaaa
+```
+
+then copy the `config.sample.py` to `config.py` and set the variables like this:
+
+```python
+playback_url = "https://bbb.example/"
+
+meeting_id = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-aaaaaaaaaaaaa"
+```
+
+and run the `main.py`.
+It should download all required files into a `data/` directory and create a `without_audio.mp4` that is just the slides and cursor and a `output.mp4` that also contains the audio.
+
 ## Limitations
 - no screen-sharing support
 - no webcam support