Added AV faq

docs/FAQ.md

 # Mongoose FAQ
 
+## My Antivirus Software reports Mongoose as a security threat
+
+Mongoose doesn't contain any malicious logic. Antivirus reports a
+[false positive](http://en.wikipedia.org/wiki/Type_I_and_type_II_errors#False_positive_error). 
+This is when certain byte sequence in Mongoose accidentally matched
+a virus/malware signature in the Antivirus database.
+
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