add potential icon for rocker-org and rocker-hub

adds a potential icon for use on rocker-org and rocker-hub.

![](https://raw.githubusercontent.com/rocker-org/rocker/feature/icon/icon/rocker.png)

Just a quick mock-up in inkscape. I didn't reuse the logo files of Docker in any way in making this, it's just
semicircles and squares, so I believe this is consistent with the [Docker Terms of Use](https://www.docker.com/legal/trademark_guidelines/), but ianal and haven't yet asked them about it.  Anyway, what do you think?