Add scalability questions

[nicorusti]

• look at overelaps with the scalability section in the CP ID Clarifications in introduction and beaconing scion-cp_I-D#46
• write some text for the research draft (e.g. how could we address scalability without rate limiting). Rest belongs to the control plane ID
• Criteria for optimizing paths while compromising with scalability @SeyedaliTaba (also see question 2.7. Operating a Path-Aware Network rfc9217 )

  A concept for path-aware network operations will need to have clear
  methods for the resolution of apparent (if not actual) conflicts of
  intent between the network's operator and the path selection at an
  endpoint. I

[juagargi]

Addressed in #10 and #13