Add hint for browser tests to README.md

Change-Id: I8c6875fe8d9c7895f2701e8e5021850b4f77a026

README.md

@@ -19,4 +19,7 @@ For the tests run:
     node_modules/.bin/grunt test
 
 ## QUnit Tests
-See https://www.mediawiki.org/wiki/Manual:JavaScript_unit_testing#Run_the_tests
+See https://www.mediawiki.org/wiki/Manual:JavaScript_unit_testing#Run_the_tests
+
+## Browser Tests
+See tests/selenium/README.md