message: github: add notice to pull request template
diff: +<!--
  +!!!ATTENTION!!!
  +
  +If you are fixing *any* crash or *any* potential security issue, *do not*
  +open a pull request in this repo. Please report the issue via emailing
  +envoy-security@googlegroups.com where the issue will be triaged appropriately.
  +Thank you in advance for helping to keep Envoy secure.
  +
  +!!!ATTENTION!!!
  +
  +-->
  For an explanation of how to fill out the fields, please see the relevant section
  in [PULL_REQUESTS.md](https://github.com/envoyproxy/envoy/blob/master/PULL_REQUESTS.md)
  