Add link to Red Hat doc to install/configure CDK

[cmoulliard]

Hi Christian,

Your instructions refer that Red Hat CDK should be installed bu there is not link to the Red Hat doc explaining to the student how to install/configure it

https://access.redhat.com/documentation/en/red-hat-enterprise-linux-atomic-host/version-7/container-development-kit-installation-guide/#installing_the_cdk_on_fedora_or_red_hat_enterprise_linux