Document update steps in cli/CHANGELOG.md

[romain-grecourt]

When a newer version of the cli is available the following message is displayed:

Version 3.0.6 of this CLI is now available.
Please see https://github.com/oracle/helidon-build-tools/blob/master/cli/CHANGELOG.md to update.

However the linked page does not explicit document how to update.
We need to make sure that when users follow the link because of the message above they find instructions on how to update.