Also update doc

The default exchange format is application/json and currently the API is
supposed to use only this format, so simplify the communication.

The API shall be simple, yet powerful. Introspection seemed like cool idea, but
is not required.

There are limited amount of endpoints. There are few parameters. Those things
shall be documented.

Backward compatibility is easy. Adding new endpoint or parameter shall be always
optional.

In case of rewriting versioning will be used.