code: public AddOn createPlanAddOn(final String planCode, final AddOn addOn) {
          return doPOST(Plan.PLANS_RESOURCE +
                        "/" +
                        planCode +
                        AddOn.ADDONS_RESOURCE,
                        addOn, AddOn.class);
      }
docs: Create an AddOn to a Plan
  <p>
  
  @param planCode The planCode of the {@link Plan } to create within recurly
  @param addOn    The {@link AddOn} to create within recurly
  @return the {@link AddOn} object as identified by the passed in object