doc: add security steward on/offboarding steps

doc/guides/security-steward-on-off-boarding.md

@@ -0,0 +1,22 @@
+# Security Steward Onboarding/OffBoarding
+
+## Onboarding
+
+* Confirm the new steward agrees to keep all private information confidential
+  to the project and not to use/disclose to their employer.
+* Add them to the security-stewards team in the GitHub nodejs-private
+  organization
+* Add them to the standard team in H1 using this
+  [page](https://hackerone.com/nodejs/team_members);
+* Add them as managers of the
+  [nodejs-sec](https://groups.google.com/g/nodejs-sec/members) mailing list.
+
+## Offboarding
+
+* Remove them from security-stewards team in the GitHub nodejs-private
+  organization
+* Unless they have access for another reason, remove them from the
+  standard team in H1 using this
+  [page](https://hackerone.com/nodejs/team_members).
+* Downgrade their account to regular member in the
+  [nodejs-sec](https://groups.google.com/g/nodejs-sec/members) mailing list.