Update README.md

In this code, I have added all command-line flags to the Readme as requested here: https://codein.withgoogle.com/tasks/5387785842720768/?sp-organization=4908450618802176. I am a GCI student but waiting for another task to be approved so once that is approved I will claim the above task and link this merge request. If you have any required changes needed so I can get the linked task approved, feel free to let me know now so I can do them whilst I'm waiting. I have only been able to find one flag that is not defined, could you guide me to find the others if there are more.