message: Add comment about serde feature
diff: @@ -21,6 +21,7 @@ hex = "0.3.2"
  # Dependencies for caching.
  [dependencies.serde]
  version = "1.0"
  +# This feature is required for serde to support serializing/deserializing reference counted pointers (e.g. Rc and Arc).
  features = ["rc"]
  [dependencies.serde_derive]
  version = "1.0"
  