(maint) document breaking changes

In the CHANGELOG

Author: tkishel

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Major Release 6.0.0
 
+This is a major release as it drops support for Puppet 4.x and versions of PE
+based upon Puppet 4.x. If you are using those unsupported versions of PE, 
+continue to use version 5.3.0.
+
+Refer to the README for limitations regarding the collection of remote 
+PuppetDB metrics, and a workaround.
+
+If using this module with the Puppet Metrics Dashboard,
+upgrade to version 2.2.0 or later of that module.
+
 ## Improvements
  - Switch from the v1 to v2 Metrics API for additional metrics (for PuppetDB)
  - Collect ACE and Bolt service metrics
@@ -18,6 +28,11 @@
  - Update to PDK 1.16
  - Update documentation
 
+## Changes
+ - Drop support for Puppet 4.x and versions of PE based upon Puppet 4.x
+ - Use v2 Metrics API for additional metrics for PuppetDB
+ - Change tags to system metrics
+
 # Minor Release 5.3.0
 
 ## Improvements