Add dast usage

[tim-thacker-nullify]

Description

Add instructions to the README for running DAST scans.

Checklist

• [ ] I have added one of the patch, minor, major or no-release labels
• [ ] I have linked an issue from this repository using the Development option
• [ ] I have performed a self-review of my own code
• [ ] I have checked for redundant or commented out code
• [ ] I have commented my code where I can't make it self-documenting
• [ ] I have made corresponding changes to the documentation
• [ ] I have added any appropriate tests