API conventions: add note about when struct fields should be pointer

Signed-off-by: Stefan Büringer [email protected]

CONTRIBUTING.md

@@ -338,8 +338,10 @@ This project follows the [Kubernetes API conventions](https://github.com/kuberne
 
 Optional fields have the following properties:
 * An optional field MUST be marked with `+optional` and include an `omitempty` JSON tag.
-* Fields SHOULD be pointers if the nil and the zero values (by Go standards) have semantic differences.
-  * Note: This doesn't apply to map or slice types as they are assignable to `nil`.
+* Fields SHOULD be pointers if there is a good reason for it, e.g:
+  * the nil and the zero values (by Go standards) have semantic differences.
+    * Note: This doesn't apply to map or slice types as they are assignable to `nil`.
+  * the field is of a struct type and would show up as an empty object after marshalling (e.g. `kubectl get`) 
 
 #### Example
 
@@ -439,4 +441,4 @@ As of today there are following OWNERS files/Owner groups defining sub areas:
 - [Infrastructure Provider Docker (CAPD)](test/infrastructure/docker)
 - [Test](test)
 - [Test Framework](test/framework)
-- [Docs](docs)
+- [Docs](docs)