Adding security file

This file:
- Shows up in the GitHub UI under the new security tab
- Points to our common process documented in the community repo.
  This follows the same pattern we use for the code of conduct

Signed-off-by: Matt Farina <matt@mattfarina.com>

SECURITY.md

@@ -0,0 +1,3 @@
+# Helm Security Reporting and Policy
+
+The Helm project has [a common process and policy that can be found here](https://github.com/helm/community/blob/master/SECURITY.md).