github: update bug template to link to the CVE process (#4131)

.github/ISSUE_TEMPLATE/bug.md

@@ -1,12 +1,17 @@
 ---
 name: Bug Report
-about: Create a report to help us improve
+about: Report a non-security bug.  For suspected security vulnerabilities or crashes, please use "Report a Security Vulnerability", below.
 labels: 'Type: Bug'
 
 ---
 
-Please see the FAQ in our main README.md, then answer the questions below before
-submitting your issue.
+NOTE: if you are reporting is a potential security vulnerability or a crash,
+please follow our CVE process at
+https://github.com/grpc/proposal/blob/master/P4-grpc-cve-process.md instead of
+filing an issue here.
+
+Please see the FAQ in our main README.md, then answer the questions below
+before submitting your issue.
 
 ### What version of gRPC are you using?