docs: add Spanish README

[webbrain-one]

Adds README.es-ES.md alongside the original documentation.

[AlexMili]

Thanks! Two small things.

There are two blank lines at the top of the file, before **Reachable** verifica.... The English one starts straight in, can you match it?

Line 18, I think "dominios de estacionamiento" reads more like a car park. I believe the usual term for parked domains is "dominios aparcados".

Rest looks good. I'll wire up the links between the two READMEs after merge.