message: add mapdir pr to changelog
diff: @@ -6,6 +6,7 @@ Blocks of changes will separated by version increments.
  ## **[Unreleased]**
  +- [#470](https://github.com/wasmerio/wasmer/pull/470) Add mapdir support to Emscripten, implement getdents for Unix
  - [#467](https://github.com/wasmerio/wasmer/pull/467) `wasmer_instantiate` returns better error messages in the runtime C API
  - [#463](https://github.com/wasmerio/wasmer/pull/463) Fix bug in WASI path_open allowing one level above preopened dir to be accessed
  - [#461](https://github.com/wasmerio/wasmer/pull/461) Prevent passing negative lengths in various places in the runtime C API
  