Extend README.md for northbound service
Extend README.md for northbound service

Add details about passing configuration options into northbound service.

Closes #40

telstra/open-kilda,telstra/open-kilda,telstra/open-kilda,telstra/open-kilda,nikitamarchenko/open-kilda,carmine/open-kilda,telstra/open-kilda,nikitamarchenko/open-kilda,carmine/open-kilda,jonvestal/open-kilda,nikitamarchenko/open-kilda,nikitamarchenko/open-kilda,nikitamarchenko/open-kilda,jonvestal/open-kilda,carmine/open-kilda,jonvestal/open-kilda,carmine/open-kilda,telstra/open-kilda,jonvestal/open-kilda,carmine/open-kilda,jonvestal/open-kilda,carmine/open-kilda,telstra/open-kilda,nikitamarchenko/open-kilda