add discountGroups to ApiRoot interface

Author: jenschude

commercetools/commercetools-sdk-java-api/src/main/java/com/commercetools/api/client/ProjectApiRoot.java

@@ -308,6 +308,7 @@ public ByProjectKeyProductTailoringRequestBuilder productTailoring() {
         return with().productTailoring();
     }
 
+    @Override
     public ByProjectKeyDiscountGroupsRequestBuilder discountGroups() {
         return with().discountGroups();
     }

commercetools/commercetools-sdk-java-api/src/main/java/com/commercetools/api/client/ProjectScopedApiRoot.java

@@ -98,5 +98,7 @@ public interface ProjectScopedApiRoot {
 
     ByProjectKeyProductTailoringRequestBuilder productTailoring();
 
+    ByProjectKeyDiscountGroupsRequestBuilder discountGroups();
+
     <R> R with(Function<ProjectApiRoot, R> op);
 }