Update all.yml to add P19

Author: nataled

principles/all.yml

@@ -44,6 +44,9 @@ principles:
 - id: fp-016-maintenance
   layout: principle
   title: Maintenance (principle 16)
+- id: fp-019-term-stability
+  layout: principle
+  title: Stability (principle 19)
 - id: fp-020-responsiveness
   layout: principle
   title: Responsiveness (principle 20)