docs: add standardized CONTRIBUTING.md

[John15321]

Summary

This PR adds a standardized CONTRIBUTING.md file that:

• Welcomes new contributors
• Links to the main Flatcar CONTRIBUTING guide
• Provides a section for repo-specific guidelines

This change is part of a batch update across all Flatcar repositories to ensure consistent contribution documentation.

Ref: flatcar/Flatcar#1865

[dongsupark]

I am not sure if we need to add the file like this PR, because this repo seems unused at the moment.
The only code included is packet.ipxe, definitely unused since the Equinix Metal's sunset.

Let's not merge it for now.

[John15321]

I am not sure if we need to add the file like this PR, because this repo seems unused at the moment. The only code included is packet.ipxe, definitely unused since the Equinix Metal's sunset.

Let's not merge it for now.

@dongsupark Thank you for looking into this!
Even though the repo might not be used at all I would still like to merge it as it simply will work as a pointer for someone looking into Flatcar (possible for the first time) and point them correctly to the main document. Additionally having all the repos (or at least most) have this change applied would make future similar org-wide changes easier with fewer exceptions