Update changelog and readme

Author: mthh

CHANGELOG.md

@@ -5,6 +5,8 @@
 - 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.
+
 - Add `x_origin`, `y_origin`, `x_step` and `y_step`
   attributes to `ContourBuilder` struct. They can be set using the *builder pattern*, before calling
   the `contours` method.

README.md

@@ -107,7 +107,6 @@ let res = c.contours(&[
 println!("{:?}", res[0].to_geojson()); // prints the GeoJSON representation of the first contour
 ```
 
-```rust
 __*Output:*__
 ```rust
 Feature {