add testflight invitation request mail

README.md

@@ -54,7 +54,9 @@ Now build and run - e.g. by pressing Cmd-r - or click on the triangle at the top
 
 ## Changelog
 
-Betas are distributed via Testflight
+Betas are distributed via Testflight.
+
+Please [send us a mail](mailto:ios@getdelta.org?subject=Testflight%20invite%20request&body=Hello,%0A%0APlease%20send%20me%20a%20Testflight%20invite%20for%20Deltachat%20iOS) if you also want to test the app.
 
 ### Beta 3
 upcoming (not released)