doc: add ruby-advisory-db to the release checklist

[skip ci]

CONTRIBUTING.md

@@ -370,7 +370,10 @@ A quick checklist:
 - [ ] run `scripts/build-gems` and make sure it completes and all the tests pass
 - [ ] `for g in gems/*.gem ; do gem push $g ; done`
 - [ ] create a release at https://github.com/sparklemotion/nokogiri/releases and provide sha2 checksums
-- [ ] if security-related, email [email protected] and [email protected]
+- if security-related,
+  - [ ] publish a GHSA
+  - [ ] email [email protected] and [email protected]
+  - [ ] submit a PR to https://github.com/rubysec/ruby-advisory-db
 - [ ] update nokogiri.org
 - [ ] bump `lib/nokogiri/version/constant.rb` to a prerelease version like `v1.14.0.dev`