Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>

The doc build tools are not available in Cassini distro. Therefore,
update the README.md to generate the docs using `sphinx-build`.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>

Add Cassini introduction and building instructions.
Add Cassini distro license details.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>
Signed-off-by: Richard Neill <richard.neill@arm.com>
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: I631f04d4325f70a2d81bf4567111e5d7d98cfb99