documentation: double readme file and outdated list of translations

Created by: Danfro

1. The main folder contains a README and a README.md file. I would guess the first one should be deleted, right?
2. The doc for translations i18n.md does list a few languages as "supported". But the list of .po files is much longer. I assume this list is outdated? Maybe drop the list since maintaining it may be a lot of manual work and rather link to the /po folder of the repo for supported languages? I know, that would drop the reference of the translators. Maybe a link to contributors might cover that to some extend?