Document true level of replay

[vaastav]

• Different levels of replay:
  1.0 : record schedule and just replay schedule (e.g., do the original
  file.read during replay) -- smallest log
  1.5 : record schedule and hash of events (check consistency during
  replay) -- medium log (can detect replay inconsistency)
  2.0 : replay schedule and data -- huge log

We are aiming for level 1.5 but we are currently at 1.