Add Transifex config.

.tx/config

+[main]
+host = https://www.transifex.com
+
+[briar.briar-gtk-pot]
+file_filter = briar-gtk/po/<lang>.po
+minimum_perc = 0
+source_file = briar-gtk/po/briar-gtk.pot
+source_lang = en
+type = PO
+