Add changes entry

Author: bobzhang

Changes

@@ -30,6 +30,10 @@ Standard library:
  slower.
  (Alain Frisch)
 
+- GPR#473: Provide `Sys.backend_type` so that user can write backend-specific
+ code in some cases (for example, code generator).
+ (Hongbo Zhang)
+
 Build system:
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