Adding reference to an actively maintained Spanish translation

[SergioRibera]

Reference to the Spanish-language book maintained by the Spanish-language community @RustLangES

[Phosphorus-M]

Thanks @SergioRibera for this PR.
Hi @carols10cents, could you please help us by providing a review? This is important for us as it helps us grow as a community and give us some visibility.
We are currently undertaking other projects such as comprehensive rust maintained by @mgeisler and having an official reference assists us in unifying terminology translations
We are quite active in all aspects related to gathering/generating content in Spanish.
Related with #661

[chriskrycho]

Thank you!