Replace references to "read-cache" with references to git-read-tree in the
documentation. I chose that because reference say "see read-cache about
stages", and stages are explained in git-read-tree.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> Signed-off-by: Junio C Hamano <junkio@cox.net>