Clarify 'not-prose' usage when using prefixes (#399)

* Update README with 'not-prose' usage clarification

* Update README.md

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Co-authored-by: Jordan Pittman <[email protected]>

Author: willmruzek

README.md

@@ -245,6 +245,8 @@ If you have a block of markup embedded in some content that shouldn't inherit th
 
 Note that you can't nest new `prose` instances within a `not-prose` block at this time.
 
+Even when using a prefix for your utilities `not-prose` should not have a prefix.
+
 ### Adding custom color themes
 
 To customize the color theme beyond simple CSS overrides, you can use the JavaScript based theme API. To do that, use the `@config` directive: