Add ffmpeg4 installation instructions in warnings
_The documentation is not available anymore as the PR was closed or merged._ To make it warn only once, feel free to use a global counter in python - and if the warning has already been done, you don't do it again > Added the same formatting for the error message :)

nice!! thank you! 

> Oh and regarding the warning counter, you can do it in another PR maybe ?

Yes, more warnings is better then no warnings.... I'll merge when the CI passes
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)
