Document how to batch-rename then ffmpeg-process a batch of images separately

[geerlingguy]

I seem to do this a lot... so it would be nice to have it documented so I don't have to re-remember all the time:

# Rename images in current dir from `image00042.jpg` to instead start from `image00000.jpg`.
# (On macOS, `brew install rename`)
# TODO: Test this.
rename 's/\d+/sprintf("%05d", $&)/e' *.jpg

# Create timelapse.mp4 in current directory starting from `image00000.jpg`.
ffmpeg -framerate 20 -i image%05d.jpg -vf format=yuv420p timelapse.mp4

(Change framerate to suit your preferences.)