Import any manifest files in a new 'submanifests' directory into
west.yml.
This lets users define custom zephyr modules or easily override
built-in modules by dropping their own manifest files into
submanifests/some-user-file.yaml.
Provide an example and a README with links to the relevant
documentation in 'submanifests' to get users started.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
