Add UPGRADING notes

Not listing the details here, I think the RFC is a better reference
for the precise technical details.

[ci skip]

Author: nikic

UPGRADING

@@ -370,6 +370,12 @@ PHP 8.0 UPGRADE NOTES
   . It is now possible to fetch the class name of an object using
     `$object::class`. The result is the same as `get_class($object)`.
     RFC: https://wiki.php.net/rfc/class_name_literal_on_object
+  . New and instanceof can now be used with arbitary expressions, using
+    `new (expression)(...$args)` and `$obj instanceof (expression)`.
+    RFC: https://wiki.php.net/rfc/variable_syntax_tweaks
+  . Some consistency fixes to variable syntax have been applied, for example
+    writing `Foo::BAR::$baz` is now allowed.
+    RFC: https://wiki.php.net/rfc/variable_syntax_tweaks
 
 
 - Date: