Upgrade guide implementing RelativeTime behavior

[StorytellerCZ]

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:

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?

[longlho]

LGTM, can u run format:fix?