added impressum

[planthaber]

rock-robotics.org is hosted by the DFKI and needs an Impressum (by german law).

I added it in the footer, but I am still unsure about the content:

• I tried to make clear that the DFKI is not the maintainer of rock and is only hosting the site.
• Persons responsible for the contents are currently the owners of "rock-core" which can finally merge changes to the page (imo)
• it is not final, that the DFKI is willing to be named there, this is still TBD, but I wanted to start the discussion early.

@doudou @2maz @jakobs : Is it ok for you to mention you here?

[planthaber]

Got the ok, please merge.