Docs: Update PSA related description in readme

PSA specification is public so the link is added. Also, involve
missed external project folder 'lib'. And correct the expression
about supported architectures. Also some minor format fixups.

Change-Id: I87c294fd3b075aeb8d0d1285cc7242b501ab7aae
Signed-off-by: Ken Liu <ken.liu@arm.com>
1 file changed