docs: add CONTRIBUTING.md

Author: haoqunjiang

CONTRIBUTING.md

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+# `create-vue` Contributing Guide
+
+Hi! We are really excited that you are interested in contributing to `create-vue`. Before submitting your contribution, please make sure to take a moment and read through the following guide:
+
+## Repo Setup
+
+This repo uses [Git submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules) to store snapshots of the created templates after each release. So when cloning the repository, please make sure you have also initialized the submodules.
+
+To download everything including submodules, run the following command:
+
+```sh
+git clone --recursive https://github.com/vuejs/create-vue.git
+```
+
+If you have already cloned the repo but without adding the `--recursive` flag, please run `git submodule update --init --recursive` to initialize and update the submodule.
+
+This repo is a monorepo using pnpm workspaces. The package manager used to install and link dependencies must be [pnpm](https://pnpm.io/).
+
+## Pull Request Guidelines
+
+- Checkout a topic branch from a base branch, e.g. `main`, and merge back against that branch.
+- For any non-trivial new features or bug fixes, please open a issue first and have it approved before working on it.
+- Don't include the `playground` directory in the commits. It will be updated automatically after each release.