Draft changes to CAPTCHA Note - security and privacy

In fulfillment of my action from last week, I've added material on security and privacy that explicitly confines the scope of the document.

I created a new section, and added the proposed text as a subsection that precedes the existing text on security of CAPTCHAs.

Please see section 2 of the branch at
https://rawgit.com/w3c/apa/security-privacy-considerations/captcha/


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