Merge pull request #3613 from ARMmbed/fix_readme_build

README: Build info; Docs version updated

Author: 0xc0170

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/)