Revert "Remove unused top-level readme file"

This reverts commit fbf26ee1.

The readme file removed by this commit is indeed not used by ReadTheDocs
but it is the one displayed on:
https://git.trustedfirmware.org/TF-A/tf-a-tests.git/about



Change-Id: I267a247bb32c570e254553d65b93bf70cdaf76e4
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>