Add information around translations

Credits of those texts go to:
* https://code.briarproject.org/briar/briar/-/blob/4e5f2e31df85cb6d56559bb99bc921d322f6a1af/TRANSLATION.md
* !65 (comment 41992)

Related to #65

TRANSLATION.md

+Translations for this project are managed through Transifex:
+
+https://transifex.com/otf/briar
+
+If you'd like to volunteer as a translator, please create a Transifex account and request to be
+added to the project's translation team. The Localization Lab has some instructions and advice for
+translators here:
+
+https://wiki.localizationlab.org/index.php/Briar
+
+#### Updating translations in Briar GTK
+
+_This section is of interest only for developers of Briar GTK_.
+
+To update translations locally, first install `transifex-client`. You can then pull updates with `tx pull`.
+
+This is how updating translations in Briar GTK works:
+
+* Transifex periodically fetches the source file from code.briarproject.org
+* Translators submit their updates via Transifex
+* We pull the updates from Transifex and commit them before each release
+* Occasionally we make minor updates locally and push them to Transifex
+
+It's recommened not to edit translations locally in Git as Transifex uses a quite simple algorithm that doesn't support this use case. You might want to make your changes through Transifex instead, by joining the respective language teams.