doc: Update packaging guidelines.

* HACKING (Submitting Patches): Mention 'git send-mail'.
* doc/guix.texi (Packaging Guidelines): More details on how to prepare a
  package module.
  (Contributing): Add reference to #guix channel on Freenode.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

HACKING

@@ -2,8 +2,9 @@
 
 #+TITLE: Hacking GNU Guix and Its Incredible Distro
 
-Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
 Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
+Copyright © 2014 Pierre-Antoine Rault <par@rigelk.eu>
 
   Copying and distribution of this file, with or without modification,
   are permitted in any medium without royalty provided the copyright
@@ -85,7 +86,11 @@ wrapping it, swallowing or rejecting the following s-expression, etc.
 Development is done using the Git distributed version control system.  Thus,
 access to the repository is not strictly necessary.  We welcome contributions
 in the form of patches as produced by ‘git format-patch’ sent to
-guix-devel@gnu.org.  Please write commit logs in the [[http://www.gnu.org/prep/standards/html_node/Change-Logs.html#Change-Logs][GNU ChangeLog format]].
+guix-devel@gnu.org.  Please write commit logs in the [[http://www.gnu.org/prep/standards/html_node/Change-Logs.html#Change-Logs][GNU ChangeLog
+format]]; you can check the commit history for examples.
+
+When posting a patch to the mailing list, use "[PATCH] ..." as a subject.  You
+may use your email client or the ‘git send-mail’ command.
 
 As you become a regular contributor, you may find it convenient to have write
 access to the repository (see below.)

doc/guix.texi

@@ -2270,6 +2270,15 @@ called @code{gnew}, you may run this command from the Guix build tree:
 Using @code{--keep-failed} makes it easier to debug build failures since
 it provides access to the failed build tree.
 
+If the package is unknown to the @command{guix} command, it may be that
+the source file contains a syntax error, or lacks a @code{define-public}
+clause to export the package variable.  To figure it out, you may load
+the module from Guile to get more information about the actual error:
+
+@example
+./pre-inst-env guile -c '(use-modules (gnu packages gnew))'
+@end example
+
 Once your package builds correctly, please send us a patch
 (@pxref{Contributing}).  Well, if you need help, we will be happy to
 help you too.  Once the patch is committed in the Guix repository, the
@@ -2784,10 +2793,10 @@ deco,,, dmd, GNU dmd Manual}).
 @chapter Contributing
 
 This project is a cooperative effort, and we need your help to make it
-grow!  Please get in touch with us on @email{guix-devel@@gnu.org}.  We
-welcome ideas, bug reports, patches, and anything that may be helpful to
-the project.  We particularly welcome help on packaging
-(@pxref{Packaging Guidelines}).
+grow!  Please get in touch with us on @email{guix-devel@@gnu.org} and
+@code{#guix} on the Freenode IRC network.  We welcome ideas, bug
+reports, patches, and anything that may be helpful to the project.  We
+particularly welcome help on packaging (@pxref{Packaging Guidelines}).
 
 Please see the
 @url{http://git.savannah.gnu.org/cgit/guix.git/tree/HACKING,