message: Update CHANGELOG for postTx -> broadcastTx change
diff: # CHANGELOG
  +## 0.23.0 (unreleased)
  +
  +- @cosmjs/cosmwasm: Rename `CosmWasmClient.postTx` method to `.broadcastTx`.
  +- @cosmjs/cosmwasm: Rename `SigningCosmWasmClient.signAndPost` method to
  + `.signAndBroadcast`.
  +- @cosmjs/launchpad: Rename `CosmosClient.postTx` method to `.broadcastTx`.
  +- @cosmjs/launchpad: Rename `SigningCosmosClient.signAndPost` method to
  + `.signAndBroadcast`.
  +- @cosmjs/launchpad: Rename `PostTx`-related types to `BroadcastTxResult`,
  + `BroadcastTxSuccess` and `BroadcastTxFailure` respectively, as well as helper
  + functions `isBroadcastTxFailure`, `isBroadcastTxSuccess` and
  + `assertIsBroadcastTxSuccess`.
  +
  ## 0.22.0 (2020-08-03)
  - @cosmjs/cli: Now supports HTTPs URLs for `--init` code sources.
  