docs: create contribution guideline

Author: ktsn

.github/CONTRIBUTING.md

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+# Contributing to Vue Class Component
+
+Thank you for contributing to Vue Class Component! To manage the process smoothly, please take a look at the following points.
+
+## Issue Reporting
+
+Please make sure that your issue is either a **bug report** or **feature request**. The other kinds of issue will be closed immediately. For usage questions, 
+please use [Stack Overflow](https://stackoverflow.com/), [the official Vue.js forum](https://forum.vuejs.org), [the official Discord server](https://chat.vuejs.org), etc.
+
+Also, please try searching existing issue before creating a new one. Your issue may exist already in an old thread.
+
+### Bug Report
+
+Please **make sure to provide [minimal and self-contained reproduction](https://new-issue.vuejs.org/?repo=vuejs/vue#why-repro)**.
+A bug report without a proper reproduction may be closed immediately.
+
+To create a reproduction, you can use on-browser playground such as JSFiddle ([template for Vue Class Component](https://jsfiddle.net/ktsn/nm55jnjk/))
+or create a GitHub repository and share its link.
+
+Please also clarify the expected behavior, actual behavior and steps to reproduce the bug to confirm it precisely.
+
+### Feature Request
+
+For feature request, please make sure to clarify **the use case** that you want to solve.
+Explaining it with actual code example or pseudo code would be useful to share your idea.
+
+## Pull Request
+
+Please do not commit the files under `dist/`. They are supporsed to be generated during the releasing process.