Update CONTRIBUTING

CONTRIBUTING.md

@@ -380,14 +380,38 @@ 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
+
+From the perspective of users that will only ever call exported interface
+type's methods but never implement an interface, this project will always
+provide stable interfaces.
+
+However for users that build implementations of exported interfaces it is
+important to understand the type of interface they are implementing as not all
+interfaces in this project are stable from an implementation perspective. The
+OpenTelemetry API is allowed to accrete functionality. This means that the
+interfaces representing this part of the OpenTelemetry API will have methods
+added to them.
+
+Implementation is only ever expected by SDK developers. These developers are
+assumed to have an added responsibility of handling these breaking changes to
+the interface and resolving them in the evolution of their SDKs.
+
+All interfaces that include the following warning in their doumentation are
+allowed to be extended with additional methods being add to them.
+
+> Warning: methods may be added to this interface in minor releases.
+
+Otherwise, stable interfaces MUST NOT be modified.
+
 ## Approvers and Maintainers
 
 Approvers: