technical doc: add a design doc for hash function transition

technical doc: add a design doc for hash function transition

This document describes what a transition to a new hash function for
Git would look like.  Add it to Documentation/technical/ as the plan
of record so that future changes can be recorded as patches.

Also-by: Brandon Williams <bmwill@google.com>
Also-by: Jonathan Tan <jonathantanmy@google.com>
Also-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>