docs: add CONTRIBUTION

CONTRIBUTING.md

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+## Contribute flows
+
+1. [Create or find an issue](#create-or-find-an-issue)
+2. [Fork this repository and setup](#fork-this-repository-and-setup)
+3. [Create a branch and make commit your changes](#create-a-branch-and-make-commit-your-changes)
+4. [Create a new Pull Request](#create-a-new-pull-request)
+
+## Create or find an issue
+
+Before your contribute, you should find an issue that you want to contribute first.  
+If you can't find an issue that you want to contribute, you should [create a new issue from here](https://github.com/ronnnnn/nilts/issues/new/choose).
+
+## Fork this repository and setup
+
+Fork this repository and clone it to your local machine.  
+
+### Setup asdf (optional)
+
+Using asdf is optional, but you **must** use SDK versions that managed by [`.tool-versions`](https://github.com/ronnnnn/nilts/blob/main/.tool-versions) file.  
+
+This project use [asdf](https://asdf-vm.com) to manage Dart and Flutter SDK versions.  
+If you want to use asdf, you should install asdf.  
+After you install asdf, you should run below command on the root of this repository.
+
+```bash
+asdf install
+```
+
+If you are not familiar with asdf, you should read [asdf documentation](https://asdf-vm.com) first.
+
+### Setup Melos
+
+This project build with [Melos](https://melos.invertase.dev).  
+You should install Melos globally.
+
+```bash
+dart pub global activate melos
+```
+
+If you are not familiar with Melos, you should read [Melos documentation](https://melos.invertase.dev) first.
+
+### Setup project
+
+You should run only `melos bootstrap` and `melos prepare` commands.
+
+```bash
+// You can use `melos bs` instead
+melos bootstrap
+melos prepare
+```
+
+## Create a branch and make commit your changes
+
+You should create a branch from `main` branch and make commit your changes.
+
+### `custom_lint`
+
+nilts is built on the top of [`custom_lint`](https://github.com/invertase/dart_custom_lint) package.  
+Read `custom_lint` documentation to understand how to debug and test lint rules you created. 
+
+- [Using the Dart debugger](https://github.com/invertase/dart_custom_lint#using-the-dart-debugger)
+- [Testing your plugins using expect_lint](https://github.com/invertase/dart_custom_lint#testing-your-plugins-using-expect_lint)
+
+### Writing rules
+
+Basically, nilts respects [writing rules of Dart's official lint rules](https://github.com/dart-lang/linter/blob/main/doc/writing-lints.md).  
+You should read it before you create a new lint rule and ensure that your lint rules and documents follows it.
+
+## Create a new Pull Request
+
+After you make commit your changes, you can create a new Pull Request.  
+You should write a description of your changes with following [template](https://github.com/ronnnnn/nilts/blob/main/.github/PULL_REQUEST_TEMPLATE.md).
+Ensure that all of checks are passed with following check list in the template.