Update Documentation for Nginx SSL Role README.MD

[darthVikes]

The certificate and certificate_key are for some reason missing from:

https://github.com/jfrog/JFrog-Cloud-Installers/blob/master/Ansible/ansible_collections/jfrog/platform/roles/artifactory_nginx_ssl/README.md

certificate: Certificate variable data; including --start cert-- to --end cert--- and everything in between.
certificate_key: Certificate Key data; including --start key-- to --end key--- and everything in between.

This was needed to get this module working successfully. These variables are checked very first in the main playbook to ensure they are there and defined.

[kammathavaran]

Acknowledging. This is a missing feature we will implement in the future.