docs: add initial documentation for Legion API to README.md

[kimitrii]

Changes Made

This PR adds the initial version of the documentation for the Legion API to the README file. It includes the following sections:

- Quick Start guide for running the API locally
- Security guidelines for reporting vulnerabilities
- Initial REST API details, including status code errors
- Error code definitions for common HTTP responses

Changes Type

• Documentation update

Checklist:

• The changes do not generate new error logs or warnings.
• I have added tests that prove the fix or new feature works as expected.
• Both new and existing tests pass locally.