Let people know how to report security bugs

Author: xzyfer

SECURITY.md

@@ -0,0 +1,10 @@
+Serious about security
+======================
+
+The LibSass team recognizes the important contributions the security research
+community can make. We therefore encourage reporting security issues with the
+code contained in this repository.
+
+If you believe you have discovered a security vulnerability, please report it at
+https://hackerone.com/libsass instead of GitHub.
+