added note for avoid using :default on exactly_one_of

Author: gencer

README.md

@@ -1299,6 +1299,8 @@ params do
 end
 ```
 
+**Note**: Avoid using `:default` option on parameters. This will act like you have multiple parameters given. This will raise `mutually exclusive` exception.
+
 #### `at_least_one_of`
 
 Parameters can be defined as 'at_least_one_of', ensuring that at least one parameter gets selected.