Update README.md

Author: facchinm

README.md

@@ -1,7 +1,11 @@
-# Arduino Core for mbed enabled devices
+# [Archived] Arduino Core for mbed enabled devices
 
 The repository contains the Arduino APIs and IDE integration files targeting a generic mbed-enabled board
 
+This repository will continue tracking mbed 5.x series and new releases will only happen in case of security bugs.
+
+For new contibutions please refer to https://github.com/arduino/ArduinoCore-mbed 
+
 ## Installation
 
 Clone the repository in `$sketchbook/hardware/arduino`