README.md: move down historical explanation about the name

The explanations about why the name was chosen are secondary compared to
the description and link to the documentation.

Some consider these explanations as good computer scientists joke, but
other see it as needlessly offensive vocabulary.

This patch preserves the historical joke, but gives it less importance
by moving it to the end of the README, and makes it clear that it is a
historical explanation, that does not necessarily reflect the state of
mind of current developers.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>