Explain the format of the expr_index.txt in the README

@DesmondYuan @Mustardburger What does the first column mean? Is it being used? The current README says to refer to loo_label.csv but I guess is outdated? 

```
901|1.5        1   0
901|1.5,HN|6   1   1
901|1.5,NT|3   1   1
901|1.5,P6|20  1   0
901|1.5,PLX|60 1   0
```