Explain the annotations.reader service

[alexislefebvre]

I had the following error:

Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: The service "Liip\TestFixturesBundle\Services\DatabaseToolCollection" has a dependency on a non-existent service "annotations.reader".

It appeared after doctrine/annotations wasn't a dependency of doctrine/persistence anymore: doctrine/persistence#197 (comment)

I didn't know where to put this note about annotations.reader but I think it should be explained in the documentation.

[javiereguiluz]

Hi Alexis. I understand your frustration in this case ... but I'm not sure we should add a note about this.

We do mention doctrine/annotations and show its installation explicitly in the article about routing (see https://github.com/symfony/symfony-docs/blob/4.4/routing.rst#creating-routes-as-annotations). The case that you mentioned is about some service using that other annotation service, so it looks like a very specific issue that I don't think we should cover in the Symfony Docs.

If others disagree, please add a comment and we'll reopen to reconsider this. Thanks!