feat(implement github login): React component + Hook for GitHub OAuth

CONTRIBUTING.md

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+# Contributing to @react-oauth
+
+Thank you for your interest in contributing to @react-oauth! This document provides guidelines and instructions for contributing.
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork: `git clone https://github.com/yourusername/react-oauth.git`
+3. Install dependencies: `yarn install`
+4. Create a branch: `git checkout -b feature/your-feature-name`
+
+## Development Workflow
+
+### Building
+
+```bash
+# Build all packages
+yarn build
+
+# Start development mode for all packages
+yarn dev
+
+# Start development mode for Google package only
+yarn dev:google
+```
+
+### Linting & Formatting
+
+We use Prettier for code formatting. Code is automatically formatted on commit via lint-staged.
+
+```bash
+# Check code formatting
+yarn prettier:check
+
+# Auto-fix formatting issues
+yarn prettier:fix
+```
+
+### Committing Changes
+
+We use [Commitizen](https://github.com/commitizen/cz-cli) for creating properly formatted commit messages following [Conventional Commits](https://www.conventionalcommits.org/):
+
+```bash
+yarn commit
+```
+
+This will guide you through creating a commit message interactively.
+
+## Code Style
+
+- We use TypeScript with strict mode enabled
+- We use Prettier for code formatting (runs automatically on commit)
+- Follow the existing code style in the repository
+- Write meaningful commit messages using Conventional Commits
+- Run `yarn prettier:fix` before committing to ensure formatting
+
+## Adding a New OAuth Provider
+
+1. Create a new package directory under `packages/@react-oauth/`
+2. Follow the structure of `packages/@react-oauth/github/`
+3. Implement the OAuth flow for your provider
+4. Update the root README.md
+5. Create a changeset: `yarn changeset`
+
+## Pull Request Process
+
+1. Run `yarn build` to ensure everything builds successfully
+2. Run `yarn prettier:check` to ensure formatting is correct
+3. Update documentation if needed
+4. Create a changeset if this is a user-facing change: `yarn changeset`
+5. Submit your pull request with a clear description
+
+## Commit Messages
+
+We follow the [Conventional Commits](https://www.conventionalcommits.org/) specification. Use `yarn commit` for interactive commit creation.
+
+**Format:**
+
+```
+<type>(<scope>): <subject>
+```
+
+**Types:**
+
+- `feat:` A new feature
+- `fix:` A bug fix
+- `docs:` Documentation only changes
+- `style:` Code style changes (formatting, etc.)
+- `refactor:` Code refactoring
+- `test:` Adding or updating tests
+- `chore:` Maintenance tasks
+
+**Scopes:**
+
+- `google` - `@react-oauth/google` package
+- `github` - `@react-oauth/github` package
+- `playground` - Playground app
+- `repo` - Repository config/tooling
+
+**Examples:**
+
+```
+feat(google): add custom button themes
+fix(github): handle popup blocked error
+docs(repo): update contributing guide
+```
+
+## Versioning
+
+This project uses [Changesets](https://github.com/changesets/changesets) for version management. If your PR includes user-facing changes:
+
+1. Create a changeset: `yarn changeset`
+2. Select the affected packages
+3. Choose the type of change (major, minor, patch)
+4. Write a summary of the changes
+5. Commit the changeset file with your changes
+
+## Questions?
+
+Feel free to open an issue for any questions or concerns.