Add link how to use subscription manager

Document URL:
https://access.stage.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/installation-guide/#rpm_installation

Section Number and Name:
3.3.1.1. Subscribing to the Current JBoss EAP 7 Repository
3.3.1.2. Subscribing to a Minor JBoss EAP 7 Repository

Describe the issue:
Both chapters should contain prerequisites section about how to make correct subscription attached. The link how to do it with subscription-manager should be provided (from official subscription-manager documentation).