Please provide a full working example

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

Documentation request.

### What is the current behavior?

Currently it's not clear how to build a form with a set of array elements, which correctly tracks statuses of the fields.
The example provided in the readme just skips this problem.

Here is an updated version of the readme's example: https://codesandbox.io/s/react-final-form-field-arrays-forked-jriin?file=/index.js

As you can see, deleting a row makes the others 'dirty'.

### What is the expected behavior?

A working example to show how to properly use the component.

For example, an array of three elements `['a', 'b', 'c']` being updated by removing the first element 'a', is treated as being actually updated in a different way:
* the last element was removed
* the first two were updated.

It seems like the following bug reports describe similar problem in different ways:

https://github.com/final-form/react-final-form-arrays/issues/145
https://github.com/final-form/react-final-form-arrays/issues/37
https://github.com/final-form/react-final-form-arrays/issues/36
https://github.com/final-form/react-final-form-arrays/issues/34
https://github.com/final-form/react-final-form-arrays/issues/33

So what is the proper way of using FieldArray?

### Sandbox Link

https://codesandbox.io/s/react-final-form-field-arrays-forked-jriin?file=/index.js

### What's your environment?

See the sandbox.

Start with the README example and the linked CodeSandbox index.js; compare them with the behavior described when removing the first array item. Review issues #145, #37, #36, #34, and #33 for the existing discussion. Done means the README contains a full working FieldArray example that correctly demonstrates field statuses after row deletion.

Written by the indexing model from the issue text.