* Add a swagger.yaml file

@balloob
I created a swagger configuration file that will help people create clients (apps / frontends) for Home Assistant more easily. Based upon this code it is even possible to generate client code for several programming languages.

I created it by hand now, so when the API changes it will need to be updated. That's why it would be better to generate this specification automatically. This is possible for API frameworks but I don't know whether it is possible for the handwritten endpoints in Home Assistant. Maybe you could assist here?

This documentation could be used to replace a part of https://home-assistant.io/developers/rest_api/.

* Added restrict parameter

* Moved swagger file to docs folder