add Buildtime Trend shield badge [skip ci]

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

Signed-off-by: Dieter Adriaenssens <ruleant@users.sourceforge.net>

README.rst

@@ -22,6 +22,10 @@ Code status
 .. image:: https://scrutinizer-ci.com/g/phpmyadmin/phpmyadmin/badges/quality-score.png?s=93dfde29ffa5771d9c254b7ffb11c4e673315035
     :target: https://scrutinizer-ci.com/g/phpmyadmin/phpmyadmin/
 
+.. image:: https://buildtimetrend.herokuapp.com/badge/phpmyadmin/phpmyadmin
+    :alt: Buildtime Trend badge
+    :target: https://buildtimetrend.herokuapp.com/dashboard/phpmyadmin/phpmyadmin
+
 .. image:: https://img.shields.io/sourceforge/dm/phpmyadmin.svg
     :target: https://www.phpmyadmin.net/downloads/