README: Update description, documentation and license information

* Give a high-level overview of what the project is about
* Inform the user how to build and access the project's documentation
* Add licensing information regarding the source code of the project

Issue-Id: SCM-5314
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
Change-Id: I2ae043eb319733506009c6980fb64818349e46fe