README: Add disclaimer discouraging solution PRs

[eyad-alkhalidy]

Because

The README needed a clear disclaimer to prevent learners from opening solution pull requests. This helps keep the exercises intact for new students and reduces the maintenance burden on the maintainers.

This PR

• Added an IMPORTANT disclaimer section to the README explaining that students should not open PRs with their exercise solutions.

Issue

Closes #380

Additional Information

N/A

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder