Updated instructions for gettext on Windows.

This is only needed for people who either want to extract message IDs or compile

message files (``.po``). Translation work itself just involves editing existing

files of this type, but if you want to create your own message files, or want to

test or compile a changed message file, you will need the ``gettext`` utilities:

* Download the following zip files from the GNOME servers

  https://download.gnome.org/binaries/win32/dependencies/

  * ``gettext-runtime-X.zip``

  * ``gettext-tools-X.zip``

  ``X`` is the version number, we are requiring ``0.15`` or higher.

* Extract the contents of the ``bin\`` directories in both files to the

  same folder on your system (i.e. ``C:\Program Files\gettext-utils``)

* Update the system PATH:

  * ``Control Panel > System > Advanced > Environment Variables``.

  * In the ``System variables`` list, click ``Path``, click ``Edit``.

  * Add ``;C:\Program Files\gettext-utils\bin`` at the end of the

    ``Variable value`` field.

test or compile a changed message file, download `a precompiled binary

installer <http://mlocati.github.io/gettext-iconv-windows/>`_.

You may also use ``gettext`` binaries you have obtained elsewhere, so long as

the ``xgettext --version`` command works properly. Do not attempt to use Django