README: add project info, documentation link, license and contact

README.md will work as the project's portal with necessary info:
- Project information
- Link to the online documentation
- How to build local version documentation
- Repository license
- Contact

Issue-Id: SCM-4372
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Ia7c2f18dc16a940a66b240653d39fffb6159e722