docs(readme): added CSSE couse by practical devsecops

[restuhaqza]

This Merge Request updates the Trainings section in the README.md file by adding information about the Certified Software Supply Chain Security Course.

Changes made:

1. Added a new entry in the Trainings section for the Certified Software Supply Chain Security Course.
2. Included a link to the course's official page.
3. Provided a brief description of the course content and its alignment with industry frameworks.

Updated section in README.md:

Trainings

• cicd-goat - Deliberately vulnerable CI/CD environment. Hack CI/CD pipelines, capture the flags.
• Certified Software Supply Chain Security Course - CSSE offers comprehensive training on identifying, validating, and mitigating software supply chain risks through hands-on exercises. Covers everything from third-party code to cloud services, aligning with industry frameworks like MITRE ATT&CK and NIST SSDF.

Motivation:
This addition enhances the value of our repository by providing information about a comprehensive training resource in software supply chain security, which is a critical aspect of AI security.

Related issues: None