docs(checkbox): Add example with error message

src/components/Checkbox/Checkbox.md

@@ -11,9 +11,25 @@
 
 ### Getting feedback for an unchecked checkbox
 
-Use the feedback attribute to give the user feedback regarding a missed checkbox by highlighting the label and checkbox element.
-
+Use the feedback and error props to give the user feedback regarding a missed checkbox by highlighting the label, checkbox element and adding an error message stating how to proceed.
 
 ```
-  <Checkbox name="terms" value="agree" label="I agree to the terms and conditions" feedback="error" />
+const message = "Please confirm you have read the terms and conditions";
+
+initialState = {
+  checked: false,
+  feedback: 'error',
+  message: message,
+};
+
+const handleCheck = (event) => {
+  if (event.target.checked === true) {
+    setState({ checked: false, feedback: undefined, message: undefined});
+  } else {
+    setState({ checked: true, feedback: 'error', message: message});
+  }
+};
+
+
+  <Checkbox name="terms" value="agree" label="I agree to the terms and conditions" feedback={state.feedback} error={state.message} onChange={handleCheck} checked={state.checked} />
 ```