Documentation: push/receive hook references.

Mention documentation pages that talk about update and
post-update hooks from git-push, because a frequently asked
question is "I want X to happen when I push" and people would
not know to look at git-receive-pack documentation until they
understand that is what runs on the other end.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-push.txt

@@ -16,6 +16,10 @@ DESCRIPTION
 Updates remote refs using local refs, while sending objects
 necessary to complete the given refs.
 
+You can make "interesting" things to happen on the repository
+every time you push into it, by setting up 'hooks' there.  See
+documentation for gitlink:git-receive-pack[1].
+
 
 OPTIONS
 -------
@@ -31,6 +35,7 @@ include::pull-fetch-param.txt[]
 	This flag disables the check.  What this means is that the
 	local repository can lose commits; use it with care.
 
+
 Author
 ------
 Written by Junio C Hamano <junkio@cox.net>

Documentation/git-receive-pack.txt

@@ -71,6 +71,10 @@ packed and is served via a dumb transport.
 	#!/bin/sh
 	exec git-update-server-info
 
+There are other real-world examples of using update and
+post-update hooks found in the Documentation/howto directory.
+
+
 OPTIONS
 -------
 <directory>::