Add a note to tw props usage

[lamualfa]

the problem is that you are still need to import twin.macro in the file that use tw props. I was confused about why the use of tw props is not working because there's no error in console. and i only realized the problem when i matching again my code and the exisiting example code.

Please add some note into the example use of tw props in README.md about that needs.

Thanks.

[ben-rogerson]

Sure, this could be a good improvement.

Make sure you check out the prop styling guide - you'll see there's a way to use the tw prop without the twin.macro import.