west.yml: allow users to easily add more modules

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>