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 <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>