Add note about this feature being available in Plasma 5.8

README.md

@@ -1,14 +1,16 @@
 KSUPERKEY
 =========
 
-ksuperkey allows you to open the application launcher in KDE Plasma
+ksuperkey allows you to open the application launcher in KDE Plasma < 5.8
 using the Super key (also known as the "Windows key"). If you hold down the
 Super key it will still act as a modifier key, allowing you to use it for
 other keyboard shortcuts.
 
 ksuperkey is a small application that runs in the background as a daemon. It
 was forked from xcape by Albin Olsson: https://github.com/alols/xcape
 
+**NOTE:** This feature is supported by default in Plasma 5.8 and above.
+
 
 Installation
 ------------