Migration Guide: Add references for migration from EAP 5 to EAP 7

Because we need to focus on migration from JBoss EAP 5 to JBoss EAP 7, but a new migration guide will be a huge effort, and may not be feasible, we need to provide information to our customers in the meantime. We should add the following the JBoss EAP 7 Migration Guide:

• A pointer the the Migration Guide for EAP 6.
• Information about Windup and the additon of rules for migration from JBoss EAP 5 - 7.
• Pointers to relevant KBase articles

See this comment for details: https://issues.jboss.org/browse/EAP7-69?focusedCommentId=13252425&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13252425