docs(contributing): add note about running gulp
docs(contributing): add note about running gulp

Closes #299

AyogoHealth/router,kyroskoh/router,shangyilim/router,rakeshbhavsar/router,shangyilim/router,IgorMinar/router,JanPietrzyk/router,rakeshbhavsar/router,excellalabs/router,excellalabs/router,AyogoHealth/router,IgorMinar/router,angular/router,kyroskoh/router,IgorMinar/router,JanPietrzyk/router,shangyilim/router