message: document tunable hyperparameters
diff: @@ -95,6 +95,28 @@ cards.
  --devices=0,1,2,3,4,5,6,7
  ```
  +### Tunable hyperparameters
  +
  +The following flags are allowed to be modified by entrants:
  +Flags that don't directly affect convergence:
  + - _all flags related to file paths & device IDs_
  + - `bool_features`
  + - `input_layout`
  + - `summary_steps`
  + - `cache_size_mb`
  + - `num_read_threads`
  + - `num_write_threads`
  + - `output_threads`
  + - `selfplay_threads`
  + - `parallel_search`
  + - `parallel_inference`
  + - `concurrent_games_per_thread`
  +
  +Flags that directly affect convergence:
  + - `train_batch_size`
  + - `lr_rates`
  + - `lr_boundaries`
  +
  ### Selfplay threading model
  The selfplay C++ binary (`//cc:concurrent_selfplay`) has multiple flags that control its
  