doc: mention v18.x openssl migration guide

Author: RafaelGSS

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