docs: Improve README with helpful contributor resources
Added a small section to the end of the README that provides useful resources for new contributors, including:

- Official Pandas cheat sheet
- Beginner tutorials
- “Good first issues” link
- Slack community link

This addition aims to encourage and guide new contributors without altering any of the existing README content.

Let me know if this fits the community guidelines — happy to adjust!
Thanks for the PR.

Since there's no issue discussing this inclusion, I don't think we necessarily need to add this at this time so closing. If interested in contributing feel free to tackle issues labeled `good first issue`