doc: Add "Adding new packages" in `HACKING'.

* HACKING (Adding new packages): New section.

HACKING

@@ -9,6 +9,50 @@ Copyright © 2012 Ludovic Courtès <ludo@gnu.org>
   notice and this notice are preserved.
 
 
+* Adding new packages
+
+Package recipes in Guix look like this:
+
+#+BEGIN_SRC scheme
+  (package
+    (name "nettle")
+    (version "2.5")
+    (source
+      (origin
+        (method http-fetch)
+        (uri (string-append "http://ftp.gnu.org/gnu/nettle/nettle-"
+                            version ".tar.gz"))
+        (sha256
+          (base32
+            "0wicr7amx01l03rm0pzgr1qvw3f9blaw17vjsy1301dh13ll58aa"))))
+    (build-system gnu-build-system)
+    (inputs `(("gnum4" ,gnum4)))
+    (propagated-inputs `(("gmp" ,gmp)))
+    (home-page
+      "http://www.lysator.liu.se/~nisse/nettle/")
+    (synopsis "GNU Nettle, a cryptographic library")
+    (description
+      "Nettle is a cryptographic library...")
+    (license "GPLv2+"))
+#+END_SRC
+
+Such a recipe can be written by hand, and then tested by running
+‘./pre-inst-env guix-build nettle’.
+
+When writing the recipe, the base32-encoded SHA256 hash of the source
+code tarball, which can be seen in the example above, can be obtained by
+running:
+
+  guix-download http://ftp.gnu.org/gnu/nettle/nettle-2.5.tar.gz
+
+Alternatively, it is possible to semi-automatically import recipes from
+the [[http://nixos.org/nixpkgs/][Nixpkgs]] software distribution using this command:
+
+  guix-import /path/to/nixpkgs/checkout nettle
+
+The command automatically fetches and converts to Guix the “Nix
+expression” of Nettle.
+
 * Porting the Guix distro on a new platform
 
 ** Introduction