Add a LICENSE and CONTRIBUTE.md file

[JulioJu]

For the future articles, I propose CC-BY-SA 4.0 . For others parts of the the website, maybe MIT could be cool, or WTFL 2.0 (what the fuck you like public license 2). ;-)

I don't know if past articles could be CC-BY-SA, because authors did not agree.

We should add a CONTRIBUTE.md that explain that.