add potential icon for rocker-org and rocker-hub
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 does is consistent with the Docker Terms of Use, https://www.docker.com/legal/trademark_guidelines/, but ianal and haven't asked them about it.  Anyway, what do you think?
Merge pull request #31 from rocker-org/feature/icon

add potential icon for rocker-org and rocker-hub