Adding PostToInsecure text.

extensions/psm-glue/res/locale/en-US/security.properties

@@ -23,5 +23,6 @@ Title=Security Warning
 MixedContentMessage=You have requested a secure document that contains some insecure information.
 LeaveSiteMessage=You have requested an insecure document. The document and any information you send back could be observed by a third party while in transit.
 EnterSiteMessage=You have requested a secure document. The document and any information you send back are encrypted for privacy while in transit.
+PostToInsecure=Although this document is secure, the information you have submitted is insecure and could be observed by a third party while in transit. If you are submitting passwords, credit card numbers, or other information you would like to keep private, it would be safer for you to cancel the submission.
 DontShowAgain=Show Me This Alert Next Time.