Update CONTRIBUTING

CONTRIBUTING.md

@@ -380,14 +380,23 @@ func NewDog(name string, o ...DogOption) Dog    {…}
 func NewBird(name string, o ...BirdOption) Bird {…}
 ```
 
-### Interface Type
+### Interfaces
 
 To allow other developers to better comprehend the code, it is important
 to ensure it is sufficiently documented. One simple measure that contributes
 to this aim is self-documenting by naming method parameters. Therefore,
 where appropriate, methods of every exported interface type should have
 their parameters appropriately named.
 
+#### Interface Stability
+
+All exported stable interfaces that include the following warning in their
+doumentation are allowed to be extended with additional methods.
+
+> Warning: methods may be added to this interface in minor releases.
+
+Otherwise, stable interfaces MUST NOT be modified.
+
 ## Approvers and Maintainers
 
 Approvers: