Update CONTRIBUTING.md to enhance development guidelines

CONTRIBUTING.md

@@ -1,37 +1,81 @@
 # How to contribute to LINE Bot SDK for Node.js
 
 First of all, thank you so much for taking your time to contribute! LINE Bot SDK
-for Node.js is not very different from any other open source projects. It will
-be fantastic if you help us by doing any of the following:
+for Node.js is not very different from any other open source projects you are aware of. It will
+be amazing if you could help us by doing any of the following:
 
-- File an issue in [the issue tracker](https://github.com/line/line-bot-sdk-nodejs/issues)
-  to report bugs and propose new features and improvements.
-- Ask a question using [the issue tracker](https://github.com/line/line-bot-sdk-nodejs/issues).
+- File an issue in [the issue tracker](https://github.com/line/line-bot-sdk-nodejs/issues) to report bugs and propose new features and improvements.
+- Ask a question using [the issue tracker](https://github.com/line/line-bot-sdk-nodejs/issues) (__Please ask only about this SDK__).
 - Contribute your work by sending [a pull request](https://github.com/line/line-bot-sdk-nodejs/pulls).
 
 ## Development
 
-You can freely fork the project, clone the forked repository, and start editing.
+### Install dependencies
 
-Here are each top-level directory explained:
+Run `npm install` to install all dependencies for development.
 
-* `lib`: TypeScript source code. You may modify files under this directory.
-* `test`: Test suites. Please add tests for modification if possible.
-* `examples`: Example projects using this SDK
-* `docs`: [VitePress](https://vitepress.dev/) markdowns for project documentation
-* `tools`: Useful tools
+### Understand the project structure
 
-Also, you may use the following npm scripts for development:
+The project structure is as follows:
 
-* `npm run test`: Run test suites in `test`.
-* `npm run format`: Format source code with [Prettier](https://github.com/prettier/prettier)
-* `npm run format:check`: Silently run `format` and report formatting errors
-* `npm run build`: Build TypeScript code into JavaScript. The built files will
-  be placed in `dist/`.
-* `npm run docs`: Build and serve documentation
+- `lib`: The main library code.
+- `test`: Test code.
+- `examples`: Example projects that use the library.
+- `docs`: [VitePress](https://vitepress.dev/) markdowns for project documentation.
+- `generator`: Custom OpenAPI generator implementation for this SDK.
 
-We test, lint and build on CI, but it is always nice to check them before
-uploading a pull request.
+### Edit OpenAPI templates
+
+Almost all API client code is generated with OpenAPI Generator based on [line-openapi](https://github.com/line/line-openapi)'s YAML files.
+Thus, you cannot edit most code under `lib` directory directly.
+
+You need to edit the custom generator templates under the `generator` directory instead.
+
+After editing the templates, run the `generate-code.py` script to generate the code, and then commit all affected files.
+If not, CI status will fail.
+
+When you update code, be sure to check consistencies between generated code and your changes.
+
+### Add unit tests
+
+We use [Vitest](https://vitest.dev/) for unit tests.
+Please add tests to the appropriate test directories to verify your changes.
+
+Especially for bug fixes, please follow this flow for testing and development:
+1. Write a test before making changes to the library and confirm that the test fails.
+2. Modify the code of the library.
+3. Run the test again and confirm that it passes thanks to your changes.
+
+### Run your code in your local
+
+You can use the example projects to test your changes locally before submitting a pull request.
+
+### Run CI tasks in your local
+
+The following npm scripts are available for development:
+
+* `npm run test`: Run test suites using Vitest.
+* `npm run format`: Format source code with [Prettier](https://github.com/prettier/prettier).
+* `npm run build`: Build TypeScript code into JavaScript. The built files will be placed in `dist/`.
+
+We test, lint and build on CI, but it is always nice to check them before uploading a pull request.
+For details on the commands executed in the CI, please refer to `.github/workflows/test.yml`.
+
+### Documentation
+This project uses two documentation systems:
+
+1. **API Reference Documentation** - Generated with [TypeDoc](https://typedoc.org/)
+   - Source is automatically generated from JSDoc comments in the code
+   - Output files are located in the `docs/apidocs` directory
+   - To generate API documentation, run: `npm run apidocs`
+
+2. **User Documentation** - Built with [VitePress](https://vitepress.dev/)
+    - Source files are located in the `docs` directory
+    - Contains guides, tutorials, and general usage information
+    - To build and serve user documentation locally, run: `npm run docs`
+
+**Please make sure your new or modified code is covered by proper JSDoc comments.**
+Good documentation ensures that contributors and users can easily read and understand how the methods and classes work.
 
 ## Contributor license agreement