Add design documentation requirements in CONTRIBUTING.md
Add design documentation requirements in CONTRIBUTING.md

telstra/open-kilda,telstra/open-kilda,jonvestal/open-kilda,jonvestal/open-kilda,telstra/open-kilda,telstra/open-kilda,telstra/open-kilda,jonvestal/open-kilda,telstra/open-kilda,jonvestal/open-kilda,telstra/open-kilda,jonvestal/open-kilda