Add guide for label management

This issue is to complete the following item from #75 

- Document how issue labels are managed. Github provides [default labels](https://docs.github.com/en/issues/using-labels-and-milestones-to-track-work/managing-labels#about-default-labels), see [an example](https://github.com/notaryproject/.github/blob/main/CONTRIBUTING.md#labels)