[doc] add doc/TRANSLATING.md

Author: ganthern

README.md

@@ -23,10 +23,15 @@ communicate securely with anyone on all your devices.
 
 </div>
 
+### Translating Tuta
+
+See [TRANSLATING.md](doc/TRANSLATING.md)
+
 ### Building
 
 See [BUILDING.md](doc/BUILDING.md).
 
-### Developing Tuta Mail
+### Developing Tuta
 
 See [HACKING.md](doc/HACKING.md).
+

doc/TRANSLATING.md

@@ -0,0 +1,7 @@
+# Translating Tuta
+
+The Tuta apps are available in more than 40 languages, most of which have been translated by volunteers.
+We are using [POEditor](https://poeditor.com) to manage the translation project. 
+Thanks to them for approving the Tuta clients for their generous open source program!
+
+If you're interested in improving one of the available language versions or even start a new one, drop us a message at [hello@tutao.de](mailto:hello@tutao.de)!