add Buildtime Trend shield badge

for other badges, see https://github.com/buildtimetrend/service#shield-badges

README.rst

@@ -20,6 +20,10 @@ Weblate
 .. image:: https://www.codacy.com/project/badge/394a22d396414d48abb6a85054f4415e
     :alt: Codacy Badge
     :target: https://www.codacy.com/public/michal_2/weblate
+    
+.. image:: https://buildtimetrend.herokuapp.com/badge/nijel/weblate
+    :alt: Buildtime Trend badge
+    :target: https://buildtimetrend.herokuapp.com/dashboard/nijel/weblate
 
 .. image:: https://api.bountysource.com/badge/tracker?tracker_id=253393
     :alt: Bountysource