Add comment about appending entries

Author: alan-j-hu

llvm/include/llvm-c/DebugInfo.h

@@ -158,6 +158,8 @@ typedef enum {
 /**
  * The kind of metadata nodes.
  */
+// NOTE: New entries should always be appended instead of matching the order
+// in Metadata.def.
 enum {
   LLVMMDStringMetadataKind,
   LLVMConstantAsMetadataMetadataKind,