git(1): show link to contributor summary page

We earlier removed a link to list of contributors that pointed to a
defunct page; let's use a working one from Ohloh.net to replace it
instead.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

Documentation/git.txt

@@ -870,7 +870,10 @@ Authors
 -------
 Git was started by Linus Torvalds, and is currently maintained by Junio
 C Hamano. Numerous contributions have come from the git mailing list
-<git@vger.kernel.org>.  If you have a clone of git.git itself, the
+<git@vger.kernel.org>.  http://www.ohloh.net/p/git/contributors/summary
+gives you a more complete list of contributors.
+
+If you have a clone of git.git itself, the
 output of linkgit:git-shortlog[1] and linkgit:git-blame[1] can show you
 the authors for specific parts of the project.