cvs-migration document: make the need for "push" more obvious

It really is an important concept to grasp for people coming
from CVS. Even if it is briefly mentioned, it is not obvious
enough to sink in.

[jc: with wording updates from J. Bruce Fields]
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/cvs-migration.txt

@@ -24,6 +24,11 @@ First, note some ways that git differs from CVS:
     single shared repository which people can synchronize with; see below
     for details.
 
+  * Since every working tree contains a repository, a commit in your
+    private repository will not publish your changes; it will only create
+    a revision. You have to "push" your changes to a public repository to
+    make them visible to others.
+
 Importing a CVS archive
 -----------------------