docs: Write a Contributing guide

.github/CONTRIBUTING.md

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+# Contributing
+
+I appreciate your considering contributing `eslint-config-moneyforward`. This document is a guide to help make your contribution easier.
+
+## Getting Started
+
+### Prerequisites
+
+- [Node.js](https://nodejs.org) (See [`.node-version`](/moneyforward/eslint-config-moneyforward/.node-version))
+
+### Installation
+
+1. Folk this repository and clone it locally
+
+   ```bash
+   git clone https://github.com/your-username/eslint-config-moneyforward.git
+   ```
+
+2. Move to the directory and install dependencies
+
+   ```bash
+   cd eslint-config-moneyforward
+   npm install
+   ```
+
+### Development
+
+The main scripts used during development are:
+
+- `npm run lint`: Runs lint on your code.
+- `npm run test`: Runs unit tests.
+
+## How to Contribute
+
+### Reporting Issues
+
+1. Check [the Issue Tracker](moneyforward/eslint-config-moneyforward/issues) for existing issues.
+2. When requesting a new issue or feature, please use [the templates](/moneyforward/eslint-config-moneyforward/issues/new/choose) and provide as much detail as possible.
+
+### Development
+
+1. Check [the Issue Tracker](moneyforward/eslint-config-moneyforward/issues), make sure if there is anything relevant to the problem you are trying to solve.
+2. Keep the repository you did folk up to date.
+
+   ```bash
+    git fetch upstream
+    git rebase upstream/main
+   ```
+
+3. Create a new branch.
+
+   ```bash
+   git switch -c feature/your-feature-name
+   ```
+
+4. Make changes to the code and run tests to make sure everything is working properly.
+5. Write a clear commit message.
+
+### Commit Messages
+
+- Commit messages should concisely describe the changes you made.
+- Commits should be split into appropriate chunks, and we recommend using [the Conventional Commits](https://www.conventionalcommits.org/) style. Below are the available Conventional Commits types:
+  - `feat`: a commit that adds new functionality.
+  - `fix`: a commit that fixes a bug.
+  - `docs`: a commit that adds or improves a documentation.
+  - `style`: changes that do not affect the meaning of the code.
+  - `refactor`: a code change that neither fixes a bug nor adds a feature.
+  - `perf`: a commit that improves performance, without functional changes.
+  - `test`: adding missing tests or correcting existing tests.
+  - `build`: changes that affect the build system or external dependencies.
+  - `ci`: changes to our CI configuration files and scripts.
+  - `chore`: other changes that don't modify src or test files.
+  - `revert`: reverts a previous commit.
+
+### Code Style
+
+- Code according to [ESlint](/moneyforward/eslint-config-moneyforward/.eslintrc.js) and Prettier rules.
+- Keep your code in a consistent style.
+
+### Testing
+
+- If you make changes, add unit tests or make sure that the existing tests pass.
+- Tests are powered by [Jest](https://jestjs.io/). When adding tests, try to increase test coverage.
+
+### Pull Requests
+
+1. Write the title of pull request in the [the Conventional Commits](https://www.conventionalcommits.org/) style. Below are the available Conventional Commits types:
+   - `feat`: a commit that adds new functionality.
+   - `fix`: a commit that fixes a bug.
+   - `docs`: a commit that adds or improves a documentation.
+   - `style`: changes that do not affect the meaning of the code.
+   - `refactor`: a code change that neither fixes a bug nor adds a feature.
+   - `perf`: a commit that improves performance, without functional changes.
+   - `test`: adding missing tests or correcting existing tests.
+   - `build`: changes that affect the build system or external dependencies.
+   - `ci`: changes to our CI configuration files and scripts.
+   - `chore`: other changes that don't modify src or test files.
+   - `revert`: reverts a previous commit.
+2. Create a pull request and include the following information:
+   - Description of the change
+   - Purpose of the change
+   - Relevant issue number (if any)
+
+## License
+
+This project is based on [MIT License](/moneyforward/eslint-config-moneyforward/LICENSE).