[css-device-adapt] note/warning about not restricting zoom

[patrickhlauke]

as per https://lists.w3.org/Archives/Public/www-style/2016Sep/0115.html and in light of the addition of a warning about the accessibility/usability concerns of preventing users from being able to zoom in the HTML spec with reference to the viewport meta (see w3c/html@29b8934 and my subsequent tweak w3c/html#607), i'm wondering if there's any mileage in me submitting a proposed PR with an initial draft note along the same lines for the CSS Device Adaptation spec? /cc @frivoal @stevefaulkner

[frivoal]

@patrickhlauke Yes, a PR would be great.