Translating the manual

[Julien Lepiller]

Hi Guix!

I've just pushed three patches to help us translate the manual!

For developpers, nothing has changed, you can commit changes to the
English manual without worrying about other languages.

For translators, we must still put the .pot files on the TP before you
can send translations there. Of course, in the you can also start a
translation by using the .pot files present in the guix repository
(po/doc/guix.pot and po/doc/contributing.pot).

For maintainers, pot files can be updated by running "make
doc-pot-update" from the top directory of the guix repository.

As an example, I've also added a French translation for the manual, with
a complete translation of the first two pages (introduction) and the
contributing section.