hook: add a simple first example

Add a simple example that replaces an outdated example
that was removed. This ensures that there's at the least
a simple example that illustrates what could be done
using the hook just by enabling it.

Also, update the documentation.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>