Add CONTRIBUTING.md with development guidelines

[jozefizso]

Description

The repository has a CODE_OF_CONDUCT.md but lacks a CONTRIBUTING.md file with development guidelines. This is important for v1.0.0 release to help contributors understand the development workflow.

What to Include

A CONTRIBUTING.md should document:

Development Setup

• Prerequisites (Node.js version, npm)
• How to clone and install dependencies
• How to build the action locally

Development Workflow

# Install dependencies
npm install

# Make changes to src/index.js

# Build the action
npm run build

# Test locally (if applicable)
# ...

Testing

• How to test changes locally
• How the verify workflow works
• Expected CI checks before merge

Submitting Changes

• Branch naming conventions (if any)
• Commit message format
• Pull request process
• Link to CODE_OF_CONDUCT.md

Release Process

• How versions are managed
• How tags are created
• Build and dist/ commit requirements

Priority

Should be completed before v1.0.0 release to establish contribution standards.

References

• GitHub's contributing guidelines guide
• Examples from similar actions: actions/setup-node