Update README.md according to refactoring

[prothen]

Document available operation modes

• Autopilot (Setpoint position)
• External Force (Decoupled DataRead based position measurements)
• External Force (request / response)
• External Force (Threaded receiver background service)

Document required

• Setup process
  1. Call connect()
  2. Activate mode
     • activate_autopilot()
  3. Run loop and check actuation_stream_is_active -> Enables / Disables autopilot, respectively external force scaling
     • This corresponds to the idle condition and allows disengagement after actuation streams are lost
  4. Call exit() on exception to terminate potential background services and leave interface with disabled autopilot and zero force control vector