Documentation: reorganize cvs-migration.txt

Documentation: reorganize cvs-migration.txt

Modify cvs-migration.txt so it explains first how to develop against a
shared repository, then how to set up a shared repository, then how to
import a repository from cvs.  Though this seems chronologically
backwards, it's still readable in this order, and it puts the more
commonly needed material closer to the front.

Remove the annotate/pickaxe section; perhaps it can find a place elsewhere
in the future.  Remove most of the "why git is better than cvs" stuff from
the introduction.

Add some minor clarifications, including two that have come up several
times on the mailing list:

	1. Recommend committing any changes before running pull.
	2. Note that changes must be commited before they can be pushed.

Update the clone discussion to reflect the new --use-separate-remotes
default, and add a brief mention of git-cvsserver.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>