Migrate GDScript design guidelines to the contributing docs.

[Ivorforce]

I randomly found this, and think it's better hosted in the contributing docs, with the other guidelines.

Corresponding contributing docs PR: godotengine/godot-contributing-docs#53

The rest of the article probably deserves a makeover as well, but this is out of scope for what I'm trying to do.

[dalexeev]

I'm not sure if this section should be removed individually. If anything should be removed from here, I think we should move the entire content to contributing docs and leave only a link in the README.

[Ivorforce]

I'm not sure if this section should be removed individually. If anything should be removed from here, I think we should move the entire content to contributing docs and leave only a link in the README.

I agree we could move it, but I think the rest of the text should be on the docs and not the contributing docs.
The difference being that we are supposed to host guidelines for contributors on the contributing docs, and technical information should be on the docs (especially if it's version sensitive).