Update ngspice regression tests README to include information about how to interpret the results

## Expected Behavior

Below is a good start - needs further update in the future.
https://github.com/mabrains/globalfoundries-pdk-libs-gf180mcu_fd_pr/blob/main/models/ngspice/testing/README.md

## Actual Behavior

https://github.com/google/globalfoundries-pdk-libs-gf180mcu_fd_pr/blob/main/models/ngspice/testing/README.md

## Specifications

- Version:
- Platform: