Add new error message to readme

Author: Tim Mendoza

README.md

@@ -113,6 +113,22 @@ POST /token
 <td> <b>Status</b> </td> <td> <b>Response</b> </td>
 </tr>
 
+<tr>
+<td> 400 </td>
+<td>
+
+```json
+{
+    "error": {
+      "message": "missing user_identity",
+      "explanation": "The user_identity parameter is missing."
+    }
+}
+```
+
+</td>
+</tr>
+
 <tr>
 <td> 401 </td>
 <td>