Improve CRL configuration documentation

[dcorbacho]

trafficstars

The SSL documentation https://www.rabbitmq.com/ssl.html could be extended to explain how CRL must be configured, as it seems confusing for non-Erlang users.

See question on serverfault: http://serverfault.com/questions/752233/rabbitmq-crl-configuration/769738#comment970056_769738

Thanks @legoscia!

[JAORMX]

Would very much appreciate this.