Re-add .tx/config

This reverts commit 81a8a03. While .tx can certainly be generated by
infra jobs on the fly, this helps people that want to work on
translations with (local) tools instead of Transifex web-interface.

Change-Id: I77ed05e929cc7500ab4ab9a46e96eb6e1384e7a7
Merge "Re-add .tx/config"