WIP: Readme.md: Add know how for update it from Weblate

Hello guys,

I have improved current Readme.md and added know how, how we or others can update the translations, which were done in Weblate, so they will be able to push it into release.

Štěpán, in future, will tell me more about it, so once he would tell me, I'll document it and will do another merge request.