Uppdate French translation and add HOWTO for translators

As it took me some time to figure out how to generate an up-to-date `.pot` file in order to integrate new source strings in the revised translation, I took the liberty of creating a dedicated HOWTO about the translation process (based on https://github.com/rmarquis/pacaur/blob/master/po/HOWTO, which I had previously used to contribute to this project). Feel free to include it or not, modify it... :relaxed:

I just saw commit 2e4517f409cc888df8ab249c90077ef3a2449bae. Made me understand what this precise string was for (maybe the occasion for a translation comment), however I didn't change the litteral translation. But if you prefer I can do another PR, taking it into account and putting my contact info there.