doc: add contributing guide

.github/CONTRIBUTING.md

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+# Vue.js Contributing Guide
+
+Hi! I'm really excited that you are interested in contributing to Vue.js. Before submitting your contribution, please make sure to take a moment and read through the following guidelines:
+
+- [Pull Request Guidelines](#pull-request-guidelines)
+- [Development Setup](#development-setup)
+
+## Pull Request Guidelines
+
+- The `master` branch is just a snapshot of the latest stable release. All development should be done in dedicated branches.
+
+- Work in the `src` folder and **DO NOT** checkin `dist` in the commits.
+
+- It's OK to have multiple small commits as you work on the PR - GitHub will automatically squash it before merging.
+
+- Make sure `npm test` passes. (see [development setup](#development-setup))
+
+- If adding a new feature:
+  - Provide a convincing reason to add this feature. Ideally, you should open a suggestion issue first and have it approved before working on it.
+
+- If fixing bug:
+  - If you are resolving a special issue, add `(fix #xxxx[,#xxxx])` (#xxxx is the issue id) in your PR title for a better release log, e.g. `update entities encoding/decoding (fix #3899)`.
+  - Provide a detailed description of the bug in the PR. Live demo preferred.
+
+## Development Setup
+
+You will need [Node.js](http://nodejs.org) **version 8+**
+After cloning the repo, run:
+
+``` bash
+$ npm install
+```
+
+### Committing Changes
+
+Commit messages should follow the [commit message convention](./COMMIT_CONVENTION.md) so that changelogs can be automatically generated. Commit messages will be automatically validated upon commit. If you are not familiar with the commit message convention, you can use `npm run commit` instead of `git commit`, which provides an interactive CLI for generating proper commit messages.
+