doc/readme-zephyr:

Zephyr-port has switched to use DT node labels instead of
label properties for partition assignment.

Also usage of west is recommended way to work with the project.

Documentation was actualized to reflect these principles.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Co-authored-by: Francesco Domenico Servidio <francesco.servidio@nordicsemi.no>