Add guide on how to run a decentraland node

[maraoz]

Maybe there already is one, but I couldn't find it here: https://docs.decentraland.org/

Is this the repo? https://github.com/decentraland/catalyst
or this one? https://github.com/decentraland/kernel

I think it'd be cool if the docs page helped newbies like me understand how they can run their own node.

Thanks!