Add instructions to show how to change CONFLUENT_CURRENT to the help menu

[kkonstantine]

Currently confluent help current will print a description of the command, its output and an example, but it does not mention the way to override the default location.

Mentioning that this is achieved by setting the environment variable CONFLUENT_CURRENT should be probably added here as well.

[mjlachman]

This was not fun to have to track down. I was rebooting my local VM each day and wondering why I had to re-register my schema each time.

[jorgheymans]

Note that the instructions are wrong, setting CONFLUENT_CURRENT does not do anything you need to set confluent_current. I'm guessing this is a bug in the script.