Add a security policy

Closes #878

Author: shepmaster

SECURITY.md

@@ -0,0 +1,6 @@
+The Rust Playground follows the same security reporting procedures as
+the main Rust organization. If you have found a vulnerability in the
+Playground, please report it according to [the Rust security
+policy][policy]. Thanks!
+
+[policy]: https://www.rust-lang.org/policies/security