Add ffmpeg4 installation instructions in warnings
Adds instructions on how to install `ffmpeg=4` on Linux (relevant for Colab users).

Looks pretty ugly because I didn't find a way to check `ffmpeg` version from python (without `subprocess.call()`; `ctypes.util.find_library` doesn't work`), so the warning is raised on each decoding. Any suggestions on how to make it look nice are welcome!

This is how it looks on Colab:
![image](https://user-images.githubusercontent.com/16348744/198052412-d48018d1-4416-4aa5-9114-f7f9b4af031f.png)
