TLS options are not documented

[na6vj]

• The github main page does not mention TLS or ssl options
• The runtime --help does not mention this either

The options --ssl_key and --ssl_cert need to be documented.

Without these, secure connection attempts fail with
Exception in thread "main" com.datastax.driver.core.exceptions.NoHostAvailableException: ...
as seen in https://yugabyte.zendesk.com/agent/tickets/5066 .