maint: Mention guix-patches@gnu.org in 'HACKING'.

Fixes <http://bugs.gnu.org/25899>.

* HACKING: Mention guix-patches@gnu.org.

HACKING

@@ -2,7 +2,7 @@
 
 #+TITLE: Hacking GNU Guix and Its Incredible Distro
 
-Copyright © 2012, 2013, 2014, 2016 Ludovic Courtès <ludo@gnu.org>
+Copyright © 2012, 2013, 2014, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 Copyright © 2015, 2017 Mathieu Lirzin <mthl@gnu.org>
 Copyright © 2017 Leo Famulari <leo@famulari.name>
 
@@ -25,8 +25,10 @@ convenient.  When you deem it necessary, feel free to ask for it on the
 mailing list.  When you get commit access, please make sure to follow the
 policy below (discussions of the policy can take place on guix-devel@gnu.org.)
 
-Non-trivial patches should always be posted to guix-devel@gnu.org (trivial
-patches include fixing typos, etc.)
+Non-trivial patches should always be posted to guix-patches@gnu.org (trivial
+patches include fixing typos, etc.)  This mailing list fills the
+patch-tracking database at [[https://bugs.gnu.org/guix-patches]]; see
+"Contributing" in the manual for details.
 
 For patches that just add a new package, and a simple one, it’s OK to commit,
 if you’re confident (which means you successfully built it in a chroot setup,
@@ -48,7 +50,7 @@ You can prevent yourself from accidentally pushing unsigned commits to Savannah
 by using the pre-push Git hook called 'pre-push'. It's located at
 'etc/git/pre-push'.
 
-For anything else, please post to guix-devel@gnu.org and leave time for a
+For anything else, please post to guix-patches@gnu.org and leave time for a
 review, without committing anything.  If you didn’t receive any reply
 after two weeks, and if you’re confident, it’s OK to commit.