Add a short description of the EventFlags example

Author: fkjagodzinski

EventFlags/README.md

@@ -1,3 +1,9 @@
 # EventFlags example
 
 EventFlags usage example for Mbed OS.
+
+This is an example showing a basic usage of `EventFlags` class. One thread is
+waiting for either of the two defined flags to be set. The second thread is
+periodically setting the flags.
+
+Note that the `wait_any()` method is also used to clear the flags.