docs: relationships and links resolving in README

[christianzoppi]

This PR adds some details about the behavior of relationships and links resolving that has been introduced with the update to the V2 of the REST API as the default version in the client. This change in the client was approved and published a long time ago but I think it's better to document clearly this behaviour.

Pull request type

Internal - no Jira link

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Other (please describe): Docs

How to test this PR

If you want to double-check the behavior, you can run this script that will fetch a story that has a reference to a story, that has a reference to another story. You can then check the returned data and see the injected stories in the rel field (2 levels of nesting):

<?php
require 'vendor/autoload.php';
$resolver = new \Storyblok\RichtextRender\Resolver();
$client = new \Storyblok\Client('wI9bYKrVy9nzrrbofXMFOQtt');

$client->resolveRelations('page.rel');
$client->getStoryBySlug('home');
$data = $client->getBody();
var_dump($data);

[fernandacanto]

Great!