message: add changes to `CHANGES.md`
diff: - <https://github.com/georust/gdal/pull/250>
  +- Add `gdal::raster::rasterband::RasterBand::unit` as wrapper for `GDALGetRasterUnitType`
  +
  + - <https://github.com/georust/gdal/pull/271>
  +
  ## 0.12
  - Bump Rust edition to 2021
  