Add list of reserved classes to UPGRADING

Author: nikic

UPGRADING

@@ -375,6 +375,28 @@ Other language changes
   Note that this only applies to calls from an incompatible context. If class B
   extended from A the call would be allowed without any notices.
 
+* It is no longer possible to use the following class, interface and trait names
+  (case-insensitive):
+
+      bool
+      int
+      float
+      string
+      null
+      false
+      true
+
+  This applies to class/interface/trait declarations, class_alias() and use
+  statements.
+
+  Furthermore the following class, interface and trait names are now reserved
+  for future use, but do not yet throw an error when used:
+
+      resource
+      object
+      mixed
+      numeric
+
 * The yield language construct no longer requires parentheses when used in an
   expression context. It is now a right-associative operator with precedence
   between the "print" and "=>" operators. This can result in different behavior