Add note about pop prevention

[nigelmegitt]

    I think pop prevention is not something that should be scripted in the file, but just noted as an observation about mixing.

I can't imagine a file where every presented audio had to have gain animation at start and end to do this job :).
Good to know that ttm:gain can be applied to foreground - as I use all our audio non-destructively, so as a minimum, I would use a single non-animated gain value.
This is well off topic. If you agree it's important to mention, then let's have a new issue?

Originally posted by @btsimonh in #45 (comment)

[nigelmegitt]

This could be added to the Audio Mixing section proposed in #118.