adding top-level privacy note (w3c#5219)

Author: Adam Argyle

mediaqueries-5/Overview.bs

@@ -56,6 +56,19 @@ Evaluating Media Features in a Boolean Context</h4>
 	the <a>media feature</a> evaluates to true.
 	Otherwise, it evaluates to false.
 
+<h3 id='mq-prefers-security'>
+Prefers-* Media Features Security and Privacy</h3>
+
+	Issue: Information about a user can be used as an active fingerprinting vector. 
+	Analysis of impact pending, more information to be provided before spec 
+	is published.
+
+	User agents and developers implementing this 
+	specification need to be aware of this vector and take it 
+	into consideration when deciding whether to use the feature. 
+	Specifically `prefers-reduced-motion`, `prefers-color-scheme` and 
+	`prefers-reduced-data` are currently of concern for exploitation.
+
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