message: Add breaking change notice to CHANGES.md
diff: # Changes
  +
  ## Unreleased
  +* **Breaking**: Change `Feature::field` return type from
  + `Result<FieldValue>` to `Result<Option<FieldValue>>`. Fields
  + can be null. Before this change, if a field was null, the value
  + returned was the default value for the underlying type.
  + However, this made it impossible to distinguish between null
  + fields and legitimate values which happen to be default value,
  + for example, an Integer field that is absent (null) from a 0,
  + which can be a valid value. After this change, if a field is
  + null, `None` is returned, rather than the default value.
  +
  + If you happened to rely on this behavior, you can fix your code
  + by explicitly choosing a default value when the field is null.
  + For example, if you had this before:
  +
  + ```rust
  + let str_var = feature.field("string_field")?
  + .into_string()
  + .unwrap();
  + ```
  +
  + You could maintain the old behavior with:
  +
  + ```rust
  + use gdal::vector::FieldValue;
  +
  + let str_var = feature.field("string_field")?
  + .unwrap_or(FieldValue::StringValue("".into()))
  + .into_string()
  + .unwrap();
  + ```
  + * <https://github.com/georust/gdal/pull/134>
  * Add basic support to read overviews
  * BREAKING: update geo-types to 0.7.0. geo-types Coordinate<T> now implement `Debug`
  * <https://github.com/georust/gdal/pull/146>
  