message: Add benchmark execution to README
diff: -# benchmark execution
  +# Benchmark Execution
  -# Ploting
  +To execute a benchmark locally:
  +
  +```bash
  +python -m xain.benchmark.exec \
  + --group_name=abc \
  + --task_name=def \
  + --dataset=fashion-mnist-100p-iid-balanced \
  + --model=blog_cnn \
  + --R=2 \
  + --E=2 \
  + --C=0.02 \
  + --B=64
  +```
  +
  +# Plotting
  To plot final task accuracies in a group of tasks use
  