request: document permissions needed for GITHUB_TOKEN

https://github.com/benchmark-action/github-action-benchmark/blob/5112d4a6bcc0377392283d37b648acc987555249/README.md?plain=1#L420-L425

can you please document what `scopes` the token need? i.e. what APIs does it call?https://docs.github.com/en/rest/overview/permissions-required-for-github-apps?apiVersion=2022-11-28

For example, I expect for the most basic scenario it only needs

```
permissions:
contents: read
```

and with `auto-push: true` it also needs

```
permissions:
contents: write
```

at the job level