message: security: Add an email template for announcing security fix of main branch
  This proposes an email template for announcing the security fix of the main branch.
diff: @@ -167,3 +167,42 @@ Thanks,
  $PERSON (on behalf of the Envoy security team and maintainers)
  ```
  +
  +## Security Fix of Main Branch Announcement
  +
  +```
  +Subject: Security fix of Envoy main branch (that includes $GITSHORTCOMMITHASH) is now available
  +To: envoy-security-announce@googlegroups.com
  +Cc: envoy-announce@googlegroups.com, envoy-security@googlegroups.com, envoy-maintainers@googlegroups.com
  +
  +Hello Envoy Community,
  +
  +The Envoy security team would like to announce the availability of the fix for security defect(s)
  +introduced in the main branch by [$GITSHORTCOMMITHASH]($GITHUBCOMMITURL) commit. The defect(s)
  +caused by the [$GITSHORTCOMMITHASH]($GITHUBCOMMITURL) were not part of any Envoy stable releases.
  +
  +$DEFECTSSUMMARY
  +
  +<!-- Provide details on features, extensions, configuration that make it likely that a system is
  +vulnerable in practice. -->
  +
  +The CVSS score for this is [$CVSSSTRING]($CVSSURL).
  +
  +Including the [$FIXGITSHORTCOMMITHASH]($FIXGITHUBCOMMITURL) commit is encouraged to fix this issue.
  +
  +**Security fix timeline**
  +
  +1. The defect(s) introduced in [$GITSHORTCOMMITHASH]($GITHUBCOMMITURL) were landed in the main
  + branch on $ORDINALDAY of $MONTH $YEAR at $PDTHOUR PDT ($GMTHOUR GMT).
  +2. The fix [$FIXGITSHORTCOMMITHASH]($FIXGITHUBCOMMITURL) was merged into the main branch on
  + $ORDINALDAY of $MONTH $YEAR at $PDTHOUR PDT ($GMTHOUR GMT).
  +
  +**Thank you**
  +
  +Thank you to $REPORTER, $DEVELOPERS, and the $RELEASEMANAGERS for the coordination in making this
  +release.
  +
  +Thanks,
  +
  +$PERSON (on behalf of the Envoy security team and maintainers)
  +```
  