Add instructions for released application translation

[DelazJ]

Transifex is currently used to translate master branch of QGIS application. And once the release is done, translations are made for the next version.
Translation quality is also an advocate for QGIS integration in some companies. Having English words in some parts or wrong translations/typos do not make QGIS look as professional as it's. However, for people caring about the quality of that released version (particularly when using an LTR) there's no instruction on how they could fix/complete the translation of the released version.
I think we should add in one of the guides (developer's?) with a link from the contribute page a section that teaches basic steps to use Qt Linguist to update released branch, share with community and get it merged

[DelazJ]

Not needed anymore, since LTR, current release and master are all translatable now.