[DOC]Add login to registry.redhat.io to the steps listed in the user manual

Description of problem:

I read [ch.2. Installing and Running the CLI] in the user guide.

I tried it, and at the same time as installing podman, I needed to log in to registry.redhat.io using my Red Hat account.
However, that is not mentioned in the user guide.

https://access.redhat.com/documentation/en-us/migration_toolkit_for_applications/7.0/html/cli_guide/installing_and_running_the_cli

Version-Release number of selected component (if applicable):

7.0.1 

How reproducible:

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