v3.1.6

Add comments explaining TAP_GITHUB_TOKEN requirements (#51)

## [DOCS]

Add inline comments to workflow files explaining why `TAP_GITHUB_TOKEN`
(a PAT) is required:

- **auto-release.yml**: Tag pushes made with `GITHUB_TOKEN` do not
trigger other workflows. Without the PAT, the Release workflow would not
run.
- **release.yml**: The default `GITHUB_TOKEN` only has access to this
repository. A PAT is needed to push to `piekstra/homebrew-tap`.