add changelog

CHANGELOG.md

@@ -81,6 +81,14 @@ for JSON encoding.
   * Every reference of `crypto.Pubkey` in context of a `Validator` is now of type string. `GetPubKeyFromBech32` must be used to get the `crypto.Pubkey`.
   * The `Keeper` constructor now takes a `codec.Marshaler` instead of a concrete Amino codec. This exact type
   provided is specified by `ModuleCdc`.
+* (distr) [\#5610](https://github.com/cosmos/cosmos-sdk/pull/5610) Migrate the `x/distribution` module to use Protocol Buffer for state
+serialization instead of Amino. The exact codec used is `codec.HybridCodec` which utilizes Protobuf for binary encoding and Amino
+for JSON encoding.
+  * `ValidatorSlashEvents` is now a struct with `slashevents `.
+  * `ValidatorOutstandingRewards` is now a struct with `rewards`.
+  * `ValidatorAccumulatedCommission` is now a struct with `commission`.
+  * The `Keeper` constructor now takes a `codec.Marshaler` instead of a concrete Amino codec. This exact type
+  provided is specified by `ModuleCdc`.
 
 ### Improvements