Add `sub_type` to the README and notes on how to use it similar to the other columns

Should look like → https://github.com/Watts-Lab/surveyor/blob/main/README.md#survey-specifications

Code looks like →

Line 40 in c0d9093

    
           | `group_prompt` |  | The overall prompt/direction for the survey taker, will appear in the survey wherever it changes to indicate a new group of questions or kind of response needed |

@TutiGomoka can you take a look at this when you have a chance. It should be quite quick.

I guess a good thing to do at that point would be to check if there are any other good README updates we can incorporate.