Add changelog

Author: natebosch

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 2.1.3-dev
 
+- `operator ==` overrides no longer take nullable arguments. This is only
+  visible for classes that implement the interfaces defined in this package
+  which no longer need to also widen the argument type.
+
 ## 2.1.2
 
 - Fix to `Quad.copy` ([#221](https://github.com/google/vector_math.dart/issues/221))