@chriswalton007,
The following pages shows supported upgrades:
https://www.cloudera.com/documentation/enterprise/upgrade/topics/ug_upgrade_paths.html
As you have identified, it is not supported to upgrade from 5.15x to 6.0.x This is due to the way we had to maintain fixes and features in branches of 5 and 6.
Don't worry, though, as you only need to wait a while longer for 6.1 at which point you can upgrade to 6.1. Only 5.15.x to 6.0.x is blocked.
We are targeting 6.1 to be released by the end of the year, but that is a rough estimate.
For now, you can keep up with the latest 5.x release and start reading the C6 upgrade documentation. There are some major changes all over CDH and for Solr the upgrade path is complex, so it is a good idea to start planning if you are using most of the CDH products.