message: add CHANGELOG entry for attributes
diff: also knows how to deallocate itself; it supports all of the HDF5 types including
  compound types, strings and arrays.
  - Added methods to `Dataset`: `layout`, `dapl`, `access_plist`, `dcpl`, `create_plist`.
  +- Added support for attributes. The Attribute API uses the new Dataset API. Attributes
  + only supports the same operations as `hdf5` (e.g. one can not perform partial IO,
  + but must read the entire attribute at once).
  ### Changed
  