Current README content is way too esoteric for someone looking at GIT
for the first time. Instead it should provide a quick summary of what
GIT is with a few pointers to other resources.

The bulk of the previous README content is moved to
Documentation/core-intro.txt.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>