Add purpose and contribution guidelines to README

[Will-thom]

Added a section explaining the purpose of the PR Maven CLI and its usefulness for various teams. Updated project documents and contribution guidelines. moving to top of document.

Summary

Explain what changed and why.

Validation

• go test ./...
• go vet ./...
• CLI behavior tested when user-facing output changes
• Golden files updated when text output intentionally changes
• Documentation updated when contributor or user behavior changes

Scope

• This PR is focused on one issue or one clearly bounded change
• No unrelated refactors
• No generated binaries, private logs, or local cache files committed

[github-actions]

Hi @Will-thom, thank you for contributing to PR Maven CLI.

I appreciate the time you spent opening this pull request. This project is intentionally shaped for focused, reviewable open source contributions, so your input helps the tool become more useful for Java and Maven teams.

A maintainer will review the context, labels, and next steps as soon as possible. If this is your first contribution here, welcome aboard.

Useful links:

• Contributing guide: CONTRIBUTING.md
• Testing guide: docs/testing.md
• CI/CD guide: docs/ci.md

Thank you for helping build a deterministic, local-first Maven failure triage tool.