README: Document the URL to use the web-based runner.

* After starting the server with "./wpt serve" is not obvious
  where the web-based runner can be accessed. Document it.

Author: clopez

README.md

@@ -89,6 +89,11 @@ After your `hosts` file is configured, the servers will be locally accessible at
 http://web-platform.test:8000/<br>
 https://web-platform.test:8443/ *
 
+To use the web-based runner point your browser to:
+
+http://web-platform.test:8000/tools/runner/index.html <br>
+https://web-platform.test:8443/tools/runner/index.html *
+
 \**See [Trusting Root CA](#trusting-root-ca)*
 
 Running Tests Automatically