Integrate robot descriptions in individual READMEs

[bittner]

The lengthy list of robot descriptions in the main README is mostly redundant. It's out of the context and thus hard to maintain.

I believe we should move all that content into the individual README files of the single robots.

We may then use the main README better for getting started instructions, including an explanation on how to SSH into the brick and how to clone this repo or copy single files onto the SD card (issue #20 heads into a similar direction). We may also link to @rhempel's nice sound demo video, which demoes how SSH'ing into the brick looks like.