Documentation: talk a little bit about GIT_NO_REPLACE_OBJECTS

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

Documentation/git-replace.txt

@@ -44,6 +44,9 @@ $ git cat-file commit foo
 
 shows information about commit 'bar'.
 
+The 'GIT_NO_REPLACE_OBJECTS' environment variable can be set to
+achieve the same effect as the `--no-replace-objects` option.
+
 OPTIONS
 -------
 -f::