message: Add CHANGELOG entry and MIGRATING section
diff: # CHANGELOG
  +## 0.13.1 (unreleased)
  +
  +**cosmwasm-std**
  +
  +- Add the new `#[entry_point]` macro attribute that serves as an alternative
  + implementation to `cosmwasm_std::create_entry_points!(contract)` and
  + `cosmwasm_std::create_entry_points_with_migration!(contract)`. Both ways are
  + supported in the 0.13 series.
  +
  ## 0.13.0 (2020-01-06)
  **all**
  