Add freeze explanations

Author: xsawyerx

Porting/release_schedule.pod

@@ -46,9 +46,11 @@ you should reset the version numbers to the next blead series.
   2020-11-20  5.33.4 ✓       Tom Hukins
   2020-12-20  5.33.5 ✓       Max Maischein
   2021-01-20  5.33.6 ✓       Richard Leach
-  2021-02-20  5.33.7         Renee Backer
-  2021-03-20  5.33.8         Atoomic
-  2021-04-20  5.33.9         Todd Rinaldo
+  2021-02-20  5.33.7         Renee Backer   Contentious changes freeze
+  2021-03-20  5.33.8         Atoomic        User-visible changes to correctly
+                                            functioning programs freeze
+  2021-04-20  5.33.9         Todd Rinaldo   Full code freeze
+  2021-05-20  5.34.0         Sawyer X       Stable release!
 
 =head1 VICTIMS