message: clarified comment in changelog
diff: - Trait `Querier` is not `Clone` and `Send` anymore.
  - `consume_region` panics on null pointers and returns `Vec<u8>` instead of
  `StdResult<Vec<u8>>`.
  -- Added contract migration mechanism. Contracts are now also expected to expose a `migrate`
  +- Added contract migration mechanism. Contracts can now optionally export a `migrate`
  function with the following definition:
  ```rust
  extern "C" fn migrate(env_ptr: u32, msg_ptr: u32) -> u32;
  