Try placing it at the very bottom of the page. Something like "This site is in alpha/beta/whatever. Please send us feedback by email or on github."

Re the contact link, the contact us page seems clear enough now that I actually peeked at it. (The problem here isn't so much what the Guardian is doing than it is what others are doing. All too often, it's impossible to find any useful email or phone number, and taking a step to contact a company amounts to wasting your time. I can't remember the last time I bothered to look at any large company's contact us page.)