[feat] Spanish translation for docs

[SanGonzalezCarretero]

Spanish translation for Koa.js documentation

I just wanted to ask the community if translations of the docs are welcome.
In that case, I could start contributing right away with an english to spanish translation of the koa.js docs

Checklist

• [x ] I have searched through GitHub issues for similar issues.
• [x ] I have completely read through the README and documentation.

[jongleberry]

I'm not sure it's worth the effort to create and maintain. is google translate not good enough?