document: add contributing.md

Author: MeilCli

.github/CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# Contributing
+Thanks for thinking of contributing this repository.
+
+## How Can I Contribute?
+### Reporting Issue
+If you want report issue, you do not have to think deeply. Let's report according to the prepared Issue Template.
+
+### Pull Request
+If you found cleary issue solution, please create pull request. In creating pull request, you should use prepared develop environment.
+
+1. Download [Visual Studio Code](https://code.visualstudio.com/)
+1. Fork this repository and clone forked repository to your PC
+1. Checkout new branch from master
+1. Open forked repository by Visual Studio Code and install recommendation extensions
+   - `dbaeumer.vscode-eslint` and `esbenp.prettier-vscode` should be installed, used by format codes
+   - `bierner.markdown-preview-github-styles` is optional recommendation, used by preview markdown
+1. Writing codes
+1. Do debug build use `npm run build`
+1. Commit and push
+   - Do not need to run `npm run pack`, because automated by CI
+1. Create Pull Request

README.md

@@ -109,5 +109,8 @@ jobs:
 ```
 danger-action can escape path of `Gemfile`. so you can put Gemfile on no-current directory.
 
+## Contributing
+see [Contributing.md](./.github/CONTRIBUTING.md)
+
 ## License
 [MIT License](LICENSE).