Add a link to CONTRIBUTING.md in README for better discoverability

[kotaikehara]

Currently, the repository has a CONTRIBUTING.md file, but it is not linked in the README.
This makes it harder for new contributors to find contribution guidelines.

Proposal

• Add a section in the README (e.g., "Contributing") with a link to CONTRIBUTING.md.
• This will improve accessibility and encourage more contributions.

Suggested Change

In README.md, add:

# Contributing
Please read the [contributing guide](https://github.com/Mail-0/Mail-0/blob/staging/.github/CONTRIBUTING.md).