Maintenance: Add README.md about the correct workflow for translations.

i18n/README.md

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+# Zammad Translations
+
+Zammad translations are managed exclusively via [translations.zammad.org](https://translations.zammad.org/).
+
+You are welcome to contribute. Please get a free account there if you want to do so.
+
+Any pull requests modifying translation files directly will be rejected.