Update enabling-ssl.markdown to include information about the prerequisites to run the SSL install script.

Had some troubles setting up SSL for RavenDB - I was mainly fooled by the script to install SSL in RavenDB, as it did not output error messages from netsh. I think this note can help others to identify what might be wrong if they encounter any troubles. Of course letting the install script emit errors from netsh would be nice, but that must be another pull request.