Add section about important parameters

[slizzered]

In the README.md, there should be a section that talks about the parameters and their influence

• Part1: What are sane parameters for a simulation and why?
  • good values to start
  • --mse-threshold
  • --reflection=1
  • keeping --repetitions at default (why!)
  • start with a low value for --min-rays, allow high values of --max-rays
• Part2: Optimizing performance: Given that a simulation yields rather good results, how can you get it to run even faster?
  • --repetitions
  • --min-rays
  • --mse-threshold
  • --adaptive-steps
  • --ngpus
  • --parallelizaion-mode=mpi -> paralllelzation on cluster
• Part3: Optimizing quality: If the results are not what you expected, what are the important parameters for tweaking?
  • Talk about the histogram
  • --max-rays
  • --repetitions
  • --mse-threshold
  • --reflection=1
  • --spectral-distribution