Update README.md

Author: rabishah

README.md

@@ -15,3 +15,14 @@ Usage
 *      $scope.$emit('hide error');      // to hide error msg
 *      $scope.$emit('show success');    // to show success msg
 *      $scope.$emit('hide success');    // to hide success msg
+
+You need to keep all your error messages in a JSON file 'errors_list.json'. e.g.
+```
+{
+  "INVALID_DATE" : "Please enter a valid date",
+  "username_not_found" : "Sorry, This username is already taken"
+}
+```
+
+Uppercase error messages represent errors comming from server while lowercase error messages represent 
+errors from client side.