Add a user guide

[Lukas-Stuehrk]

This is a discussion starter. It's a first try to explore the structure and contents of a user guide. I think it implements #73. It uses a lot of functionality of the upcoming 1.0.0-beta.6 release, so we should not merge it before this version is released.

Please feel free to change, extend and rework this. Also, in its current form it's rather boring 🇩🇪 and we might want to make it more exciting.

Also: English is not my first language, so it could benefit from improvements of a native speaker with good writing skills.

[davegomez]

I'm looking for something like this.

My doubt is how should I document a return value when is optional?

Should I just mention it in the comment and the generator will take the Optional Type automatically setting the difference somewhere?