Web link to specific whatsapp contact

I'd like to place a link on a webpage which opens a whatsapp chat with a certain whatsapp contact. In other words: I want a "contact me by whatsapp" link to go next to the page's "Contact us my facebook/twitter/google+/etc" links. Something like:

href="whatsapp:contact=015555555555@s.whatsapp.com&message="I'd like to chat with you"