docs: update contributing guide with conventions and workflows

[j-d-ha]

Description

Update the contributing guide (or create one if it doesn't exist) to document the project's conventions and workflows for contributors.

Content to Include

1. Commit Message Conventions

• Conventional commits format requirement
• Available types: feat, fix, docs, refactor, test, chore, ci
• Scope options: host, abstractions, opentelemetry, deps, build, ci, github
• Breaking change footer format
• Examples of good commit messages

2. Pull Request Requirements

• PR title must follow conventional commits format
• Validation is enforced by CI
• Link to PR template (.github/pull_request_template.md)
• Guidelines on when to use scopes

3. Branch Naming Conventions

• Document any branch naming standards
• Main branch: main
• Feature/fix branch patterns

4. Versioning & Release Process

• Explain that versioning is automatic based on commit types
• How the release process works (Release Drafter → manual publish → automated NuGet publish)
• Pre-release vs stable releases

5. Code Style & Testing

• Link to or summarize key code style guidelines
• Testing framework and patterns (xUnit, NSubstitute, AutoFixture, AwesomeAssertions)
• C# 14 extension members syntax

6. Development Workflow

• How to set up the project locally
• How to run tests
• How to build the project

Acceptance Criteria

• Contributing guide exists (CONTRIBUTING.md or similar)
• All sections above are documented
• Guide is linked from README if not already
• Examples are provided for common scenarios
• Guide is clear and easy for new contributors to follow