Add SECURITY.md.

Inspired by Adam Baldwin's JSConf 2013 "Builders and Breakers" talk.
http://blog.liftsecurity.io/post/52010883123/security-md

SECURITY.md

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+![white hat trophy](http://b.thumbs.redditmedia.com/n0_7BYpCg_RYB1j7.png)
+
+Like all pieces of software, reddit has bugs – and it always will. Some
+of them will take the form of security vulnerabilities.
+
+If you find a security vulnerability in reddit, please privately report it to
+[[email protected]](mailto:[email protected]). We'll get back to you ASAP,
+usually within 24 hours.
+
+Once the issue is fixed, if you provide your reddit username, we'll credit your
+account with a [whitehat](http://www.reddit.com/wiki/whitehat) trophy.
+
+Thank you and good hunting.