Added codesandbox link

Author: erikras

README.md

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 Most of the popular React form libraries accept an `onSubmit` function that is expected to return a `Promise` that resolves when the submission is complete, or rejects when the submission fails. This mechanism is fundamentally incompatible with action management libraries like [`redux-saga`](https://redux-saga.js.org), which perform side-effects (e.g. ajax requests) in a way that does not let the submission function easily return a promise. React Redux Promise Listener is a potential solution.
 
+### Example
+
+[![Edit 🏁 React Final Form - Async Redux Submission](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/x71mx66z8w)
+
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