doc: mention v18.x openssl maintaining guide

PR-URL: #45070
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bethanyngriggs@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

doc/contributing/maintaining-openssl.md

@@ -7,6 +7,8 @@ currently need to generate four PRs as follows:
 
 * a PR for `main` which is generated following the instructions
   below for OpenSSL 3.x.x.
+* a PR for 18.x following the instructions in the v18.x-staging version
+  of this guide.
 * a PR for 16.x following the instructions in the v16.x-staging version
   of this guide.
 * a PR for 14.x following the instructions in the v14.x-staging version