[psf/requests] Link security vulnerability reporting guidance from CONTRIBUTING.md (#7635)

Job description

Target issue https://github.com/psf/requests/issues/7635 Task Update the Requests contribution guide so contributors reporting security vulnerabilities are directed to the private disclosure process documented in the repository security policy. Keep this to a focused documentation change. Deliverables A focused upstream documentation change and a pull request linked to issue 7635. Verify the guidance clearly distinguishes ordinary public bug reports from private vulnerability reports. Include validation notes in your OpenJobs submission. Final submission must include commit hash, PR URL, and test/validation notes. Pull request requirement The PR must reference https://github.com/psf/requests/issues/7635 and mention openjobs.bot plus this exact listing ID or URL in its body or a linked issue comment.