feat: Upgrade guide implementing RelativeTime behavior (#1374)

Adding a section on how to implement the old `FormattedRelative` behavior in v3. As per discussion in:
#1364

Another option I was thinking about was adding an example was:
```js
import React from 'react';
import PropTypes from 'prop-types';
import { selectUnit } from '@formatjs/intl-utils';
import { FormattedRelativeTime } from 'react-intl';

export const FormattedRelative = ({ value, ...props }) => {
  const selectedUnit = selectUnit(new Date(value) || Date.now());
  return <FormattedRelativeTime {...props} value={selectedUnit.value} unit={selectedUnit.unit} />;
};

FormattedRelative.propTypes = {
  value: PropTypes.oneOfType([ PropTypes.date, PropTypes.string ]).isRequired
};

export default FormattedRelative;
```

Thoughts?

docs/Upgrade-Guide.md

@@ -241,6 +241,16 @@ When we introduced `FormattedRelative`, the spec for [`Intl.RelativeTimeFormat`]
 
 Similarly, the functional counterpart of this component which is `formatRelative` has been renamed to `formatRelativeTime` and its parameters have been changed to reflect this component's props accordingly.
 
+7. Implementing `FormattedRelative` behavior
+
+You can use `@formatjs/intl-utils` to get close to the previous behavior like this:
+```js
+import { selectUnit } from '@formatjs/intl-utils';
+const {value, unit} = selectUnit(Date.now() - 48 * 3600 * 1000);
+// render
+<FormattedRelativeTime value={value} unit={unit} />
+```
+
 ### `formatMessage` now supports `ReactElement`
 
 The imperative API `formatMessage` now supports `ReactElement` in values and will resolve type correctly. This change should be backwards-compatible since for regular non-`ReactElement` values it will still return a `string`, but for rich text like the example down below, it will return a `Array<string, React.ReactElement>`: