git-jump: give contact instructions in the README

git-jump: give contact instructions in the README

Let's make it clear how patches should flow into
contrib/git-jump. The normal Git maintainer does not
necessarily care about things in contrib/, and authors of
individual components should be the ones giving the final
review/ack for a patch. Ditto for bug reports, which are
likely to get more attention from the area expert.

Signed-off-by: Jeff King <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>