Update documentation how to continue on failure

[This PR](https://github.com/anchore/scan-action/pull/94) was opened for the use case of continuing on failure to check an exit code later. But there is already a way to do this with GitHub Actions, we should document this:

```yaml
- uses: anchore/scan-action@v3
id: scan
continue-on-error: true
...
- run: exit 1
if: ${{steps.scan.outcome == 'failure'}}
```

Example: https://github.com/kzantow-anchore/sbom-action-test/blob/scan-test/.github/workflows/scan-test.yml

Review the existing documentation and the linked PR #94, then compare the proposed example with .github/workflows/scan-test.yml from the referenced workflow. Document how continue-on-error and steps.scan.outcome can be used to inspect a failed scan; done means the use case is explained with a working GitHub Actions example.

Written by the indexing model from the issue text.