Add extra sentence about testing core first (ansible#2619)

* Add extra sentence about testing core first

* implement feedback

Author: samccann

docs/docsite/rst/porting_guides/porting_guide_core_2.19.rst

@@ -29,6 +29,7 @@ with wide-ranging positive effects on security, performance, and user experience
 Backward compatibility has been preserved where practical, but some breaking changes were necessary.
 This guide describes some common problem scenarios with example content, error messsages, and suggested solutions.
 
+We recommend you test your playbooks and roles in a staging environment with this release to determine where you may need to make changes.
 
 Playbook
 ========