Added prose style guidelines
Added prose style guidelines

See https://github.com/emberjs/guides/pull/902#issuecomment-150720460

bmac/guides,bmac/guides,MarkMT/guides,szines/guides,knownasilya/guides,vikram7/guides,skaterdav85/guides,szines/guides,grapho/guides,knownasilya/guides,vikram7/guides,MarkMT/guides,skaterdav85/guides,ubuntuvim/guides,davidpett/guides,HeroicEric/guides,bmac/guides,davidpett/guides,jkarsrud/guides,HeroicEric/guides,knownasilya/guides,ubuntuvim/guides,jkarsrud/guides,greyhwndz/guides,szines/guides,tleskin/guides,skaterdav85/guides,greyhwndz/guides,MarkMT/guides,jkarsrud/guides,davidpett/guides,boztek/guides,jackfranklin/guides,nicobot/guides,tleskin/guides,HeroicEric/guides,MarkMT/guides,ubuntuvim/guides,greyhwndz/guides,bmac/guides,jackfranklin/guides,ubuntuvim/guides,skaterdav85/guides,grapho/guides,jkarsrud/guides,knownasilya/guides,boztek/guides,davidpett/guides,nicobot/guides,nicobot/guides,boztek/guides,vikram7/guides,grapho/guides,tleskin/guides,jackfranklin/guides