README: fix path to "gitcvs-migration.txt" and be more consistent

README: fix path to "gitcvs-migration.txt" and be more consistent

README suggested to look at "Documentation/gittutorial.txt" for the
tutorial and to use "man git-commandname" for documentation of each
command.

This was not consistent because the tutorial can also be available with
"man gittutorial" once git is installed, and the documentation for each
command can be available at "Documentation/git-commandname.txt" before
installing git.

This patch tries to make the description more consistent.  It also fixes
the path to the cvs-migration documentation that changed from
"Documentation/cvs-migration.txt" to "Documentation/gitcvs-migration.txt".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>