Add docs for attributes for each resource

[petems]

Right now it's a bit hard to see what attributes are available on each resource.

For example, I'm moving https://github.com/pearkes/tugboat to use droplet_kit, but it's missing a few of the attributes on the image side, and I had to dig into the code to find out. Adding docs for each attribute, using the docs from the API DigitalOcean docs.

I did some of this in #115, but it would be nice to have full coverage! 😄