Cleanup/restructure readme

[gavanderhoorn]

The current readme should be updated and restructured:

• installation instructions need to be updated
• compatibility with different ROS, OS, controller and Polyscope versions need to be made more explicit
• usage instructions need to be clarified
• references to ur_modern_driver need to be added (including an explanation of when to use it: always, except potentially for CB2 or CB1 controllers)
• licensing should be made clear (license badges?)

[gavanderhoorn]

Related: #367.

[fmauch]

Closed via #614