Add __debugInfo() to UPGRADING.

RMs: this is an UPGRADING only change, and should be safe for RCs.

Developers: please remember to update UPGRADING, otherwise there's a good
chance your shiny new feature won't be documented! (As has happened here.)

Author: LawnGnome

UPGRADING

@@ -89,6 +89,10 @@ PHP 5.6 UPGRADE NOTES
 - Added a function for timing attack safe string comparison
   (https://wiki.php.net/rfc/timing_attack)
 
+- Added the __debugInfo() magic method to allow userland classes to implement
+  the get_debug_info API previously available only to extensions.
+  (https://wiki.php.net/rfc/debug-info)
+
 - Added gost-crypto (CryptoPro S-box) hash algorithm.
 
 - Stream wrappers verify peer certificates and host names by default in