message: Add CHANGELOG note on missing ContractDetails type
diff: and `.postTx`.
  - @cosmjs/cosmwasm: Use `*PostTx*` types and helpers from @cosmjs/sdk38. Remove
  exported `PostTxResult`.
  +- @cosmjs/cosmwasm: `ContractDetails` was removed in favour of just `Contract`.
  + The missing `init_msg` is now available via the contract's code history (see
  + `getContractCodeHistory`).
  - @cosmjs/sdk38: Rename `CosmosClient.getNonce` method to `.getSequence`.
  - @cosmjs/sdk38: Remove `RestClient` class in favour of new modular `LcdClient`
  class.
  