Update changelog to make explicit the breaking changes

Author: mthh

CHANGELOG.md

@@ -2,10 +2,10 @@
 
 ### 0.7.0 (2022-09-23)
 
-- Make geojson optional, use geo-types for geometry representation
+- BREAKING: Make geojson optional, use geo-types for geometry representation
   (thanks to @michaelkirk, see #5 and #6 for details).
 
-- Rename the "value" field to "threshold" in the GeoJSON representation.
+- BREAKING: Rename the "value" field to "threshold" in the GeoJSON representation.
 
 - Add `x_origin`, `y_origin`, `x_step` and `y_step`
   attributes to `ContourBuilder` struct. They can be set using the *builder pattern*, before calling