using decorators/final form calculate with form array

👋 Hey, thanks for ~~taking an interest in~~ making 🏁 React Final Form Arrays!
After trying other form library i can definitely conclude It is near perfect library for forms, just some edges to be sorted out.

### Are you submitting a **bug report** or a **feature request**?
feature request or maybe a bug report

### What is the current behavior?

There is no specification on how to use [final-form-calculate](https://github.com/final-form/final-form-calculate) or more specifically decorators with array-form

### What is the expected behavior?
exposing decorator api to for array form

### Sandbox Link
n/a

### What's your environment?
applicable to all versions

### Other information
n/a