Add the self-study guide to the list

[Raul Fraile]

Hey,

I just published the self-study guide to prepare the certification and I was wondering if it could be added to this repository. If you let me know the section that fits best, I can open a PR.

Thanks!

[Jesus Manuel Olivas]

@raulfraile there is a Book Section https://github.com/jmolivas/symfony-certification-guide#books

I think that will be the best place to add.

Feel free to send the PR