give user an info that the screen must not be locked while recording

[Danfro]

For the app to work, it either needs to have the screen turned on all the time (no screen locking!) or background suspension needs to be enabled. There is currently no in-app info for the user about this.
Only one info in the README and in the OpenStore description.

There should be an info dialog maybe (first run? or every time when recording?) or some other way to make the user aware of this.