Update README with profiling and performance info

README.md

@@ -216,6 +216,7 @@ Changes
 -   2.4.3 - in development
     -   Improve the appearance of the DevTools panel on Firefox. \[[\#217](https://github.com/matatk/landmarks/pull/217)\]
     -   Fix a bug whereby landmark updates for background tabs pages would show up in the pop-up if it's open. \[[\#216](https://github.com/matatk/landmarks/pull/216)\]
+	-   Improve the profiling script considerably, and use the results to improve performance of landmark finding. \[[\#220](https://github.com/matatk/landmarks/pull/220)\]
     -   Fewer debug-mode log messages; tidy up metadata and improve screengrabs; bump dependencies. \[[\#218](https://github.com/matatk/landmarks/pull/218)\]
 -   2.4.2 - 29th of October 2018
     -   Fix a bug with sidebar option initialisation. \[[\#213](https://github.com/matatk/landmarks/pull/213)\]