Improve documentation with solutions to common scenarios

[tylermilner]

We should add some supporting documentation for how to deal with common scenarios like

• Customizing the JSONDecoder that's used
• How to deal with requests that don't return a response object (we have a playground for this already)

[wmcginty]

I created a Documentation/Guides and Documentation/Migrations folders in the release/2.0 branch. I've also begun an ErrorHandling guid. I think this would be a good place to build on our non-code documentation.

[wmcginty]

Is there anything else we think documentation is missing for? @tylermilner

[tylermilner]

@wmcginty I think this is good for now. Now that we have a place to put the markdown files, we can add more guides as we go forward.