Update STYLE-GUIDE.md

Mention `apiVersion` must match proto package.

Author: wora

STYLE-GUIDE.md

@@ -38,9 +38,13 @@ naming convention:
 ## Versioning
 
 When defining Kubernetes Custom Resource Definition (CRD) using
-proto3, the proto versioning must match the Kubernetes versioning,
-and the proto message type must match the CRD kind name.
-see the following example.
+`proto3`, follow the following guidelines:
+* The proto `package` name must match the Kubernetes `apiVersion`,
+  excluding the `.io` DNS suffix and reversing the DNS segment
+  ordering.
+* The proto message type must match the CRD `kind` name.
+
+#### Example
 
 The Kubernetes CRD: