Update steam deploy documentation to include InstallScript option

[EdgarMagalhaes]

Changes

This PR follows the proposed change on https://github.com/game-ci/steam-deploy and adds documentation on the new option.

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [ ] Readme (updated or not needed)
• [ ] Tests (added, updated or not needed)

[github-actions[bot]]