doc: Remove "Adding new packages" from `HACKING'.

* HACKING (Adding new packages): Remove since it's now in the manual.

HACKING

@@ -78,50 +78,6 @@ addition to that, you must not miss [[http://www.emacswiki.org/emacs/ParEdit][Pa
 directly operate on the syntax tree, such as raising an s-expression or
 wrapping it, swallowing or rejecting the following s-expression, etc.
 
-* Adding new packages
-
-Package recipes in Guix look like this:
-
-#+BEGIN_SRC scheme
-  (package
-    (name "nettle")
-    (version "2.5")
-    (source
-      (origin
-        (method url-fetch)
-        (uri (string-append "mirror://gnu/nettle/nettle-"
-                            version ".tar.gz"))
-        (sha256
-          (base32
-            "0wicr7amx01l03rm0pzgr1qvw3f9blaw17vjsy1301dh13ll58aa"))))
-    (build-system gnu-build-system)
-    (inputs `(("m4" ,m4)))
-    (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 gpl2+))
-#+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.
-
 * Submitting Patches
 
 Development is done using the Git distributed version control system.  Thus,