docs: add CONTRIBUTING.md with contribution guidelines

[256clinton]

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[vercel]

@256clinton is attempting to deploy a commit to the mukisalivingstone133-9917's projects Team on Vercel.

A member of the Team first needs to authorize it.

[RonaldRonnie]

Thanks for adding the CONTRIBUTING.md file @256clinton - this is a great step toward improving collaboration 👍

The structure and guidelines (workflow, commits, PRs, etc.) are solid.

However, the issue also requires guidance on setting up the development environment (Spring Boot ), which is currently missing (or needs to be more explicit).

Please consider adding:

• Backend setup steps (Java version, ./mvnw spring-boot:run)
• Instructions for using env.example
• Any required dependencies/tools

This will ensure new contributors can actually run the project locally, which is the main goal of this issue.

Once that’s added, this should fully meet the acceptance criteria.
Thanks :)