README: Build info; Docs version updated

Add note that mbed-cli is needed for building mbed OS; update link to the docs to point to the latest version.

Author: Bartek Szatkowski

README.md

@@ -11,7 +11,7 @@ The current release, along with a selection of previous versions are detailed he
 
 ## Getting Started for Developers
 
-We have a getting started guide for developers using mbed OS in applications:
+Please note that you will require [mbed-cli](https://github.com/ARMmbed/mbed-cli) to build mbed OS. For more details please read the getting started guide for developers using mbed OS in applications:
 
 - [Getting Started](https://docs.mbed.com/docs/mbed-os-handbook/en/latest/)