Add track description to global `README.md`

[teodutu]

Prerequisite Checklist

• [X] Read the contribution guidelines regarding submitting new changes to the project;
• [x] Tested your changes against relevant architectures and platforms;
• [x] Updated relevant documentation (if needed).

Description of changes

List the chapters of this track and what components each subchapter may be part of.

[teodutu]

Converted to a PR into the SSS fork: https://github.com/security-summer-school/essentials-security/pull/1