Documentation cvs: Clarify when a bare repository is needed

New users sometimes import a project and then immediately
try to use the imported repository as a central shared repository.
This provides pointers about setting up a bare repository for that
in the parts of the documentation dealing with CVS migration.

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-cvsserver.txt b/Documentation/git-cvsserver.txt
index 3310ae2..19da87e 100644
--- a/Documentation/git-cvsserver.txt
+++ b/Documentation/git-cvsserver.txt
@@ -133,6 +133,9 @@
 <<dbbackend,Database Backend>>. If you want to offer write access over
 SSH, the users of course also need write access to the git repository itself.
 
+You also need to ensure that each repository is "bare" (without a git index
+file) for `cvs commit` to work. See linkgit:gitcvs-migration[7].
+
 [[configaccessmethod]]
 All configuration variables can also be overridden for a specific method of
 access. Valid method names are "ext" (for SSH access) and "pserver". The