Add docs which provide details how to enable SSL

**Describe the bug**

our agent required to disable SSL verification as described [here](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#validate-server-cert). The key statement is Disabling validation is normally required if using self-signed certificates.

our Doc should ref to the following URL so we can workaround till we have a builtin support
https://nodejs.org/api/cli.html#cli_node_extra_ca_certs_file.