Added information about the socat dependency

In order for artoo or at least artoo-sphero to work the `socat` package needs to be installed on Linux based systems. This is as mentioned in hybridgroup/artoo-sphero#6 .  I don't know if it's always installed on Ubuntu - however it isn't on all Debian based systems (running Linux Mint Debian Edition) so mentioning it probably helps all Linux users.

If this is a general dependency I believe it'd be worth adding in a more common getting started place or something. This way I only added it to the sphero documentation as I know it applies here.

Cheers,
Tobi