Update README.md

Author: SpencerCurtis

README.md

@@ -35,10 +35,12 @@ Create a new playground in this repository called "CurrencyConverter"
     * Check the value of `currency` to see whether you should convert to CAD or MXN
     * Perform the conversion with the dollars passed into this method
     * Return the converted value
-6. Create a function called `convert(amountString: String) -> Double` In it, do the following:
+6. Create a function called `convert(amountString: String) -> String?` In it, do the following:
     * create a constant called `amount`. Its value should be the `amountString` that is initialized into a `Double`
     * use a `guard let` to unwrap the new `amount` constant.
     * convert the dollar amount to the expected currency (hint, you'll want to call the `convert` method you created in step 4
+7. Test your code by calling `convert(amountString` with some values. Change your `currency` to both `.cad` and `.mxn` to make sure the conversion works for both currencies.
+
 
 ## Go Further (Optional)