Add 'New contributors' section to main README

[robmarkcole]

This will document what New contributors need to know, such as info on running tests and formatting.

I also got caught out by this

[deependujha]

We have CONTRIBUTING.md for new contributors to setup the project.

However we don't have specific instructions on running tests or formatting. Feel free to make a PR.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[bhimrazy]

Closing this issue for now, as I believe CONTRIBUTING.md already covers most of the relevant information. Additionally, the section on running tests is included here:
https://github.com/Lightning-AI/litData/blob/main/CONTRIBUTING.md#how-to-add-new-tests