Add comment for potential overlapping tag nums in Cadence

Currently, Atree uses CBOR tag numbers [247, 255] and grows downwards.
Cadence uses CBOR tag numbers [128, 224] and grows upwards.  There
MUST not be any overlap.

This commit adds comment to warning about overlapping.  We can be more
proactive about this by dividing up the remaining available tag numbers
between Cadence and Atree.

NOTE: A similar comment will be added to Cadence repo at
github.com/onflow/cadence when the next atree-cadence integration
PR is opened.

storable.go

@@ -67,6 +67,11 @@ func hasPointer(storable Storable) bool {
 }
 
 const (
+	// WARNING: tag numbers defined in here in github.com/onflow/atree
+	// MUST not overlap with tag numbers used by Cadence internal value encoding.
+	// As of Oct. 2, 2023, Cadence uses tag numbers from 128 to 224.
+	// See runtime/interpreter/encode.go at github.com/onflow/cadence.
+
 	CBORTagInlinedArrayExtraData     = 247
 	CBORTagInlinedMapExtraData       = 248
 	CBORTagInlinedCompositeExtraData = 249