add some more documentation to timelapse.sh

[ckuethe]

Make it easier to run timelapse.sh -h without a valid configuration to
figure out what this script does and how to use it, especially manually.

$ ./scripts/timelapse.sh -h
Usage: timelapse.sh [-h|--help] <DATE> [<IMAGE_DIR>]
    example: timelapse.sh 20211016
    or:      timelapse.sh 20211016 /media/external/allsky

<DATE> must be of the form YYYYMMDD.

<IMAGE_DIR> defaults to '${ALLSKY_IMAGES}', but may be overriden. In that case
<DATE> must exist inside <IMAGE_DIR>, eg. '/media/external/allsky/20211016'.

Produces a movie in <IMAGE_DIR>/<DATE>/allsky-<DATE>.mp4
eg. /home/ckuethe/allsky/images/20211016/allsky-20211016.mp4
or  /media/external/allsky/20211016/allsky-20211016.mp4