add yet incomplete draft for middleware interface

@gerkey @esteve @tfoote @wjwwood fyi the current draft for the middleware interface whitepaper. 

I still have to write the last part describing the actually implemented architecture.
This should probably either be:

```
Depending on the middleware implementation the extra conversion can be avoided by implementing serialization functions from, as well as deserilization functions to, the ROS messages.
```

Or

```
Depending on the middleware implementation the extra conversion can be avoided by implementing serialization functions from ROS messages as well as deserilization functions to the ROS messages.
```