Add a "How to Contribute" page

Author: haoqunjiang

How to Contribute.md

@@ -0,0 +1,9 @@
+
+# Contributing to Vue CLI
+
+There are many ways to contribute to the Vue CLI project: logging bugs, submitting pull requests, reporting issues, and creating suggestions.
+
+After cloning and building the repo, check out the [issues list](https://github.com/vuejs/vue-cli/issues?utf8=%E2%9C%93&q=is%3Aopen+is%3Aissue).
+* Issues labeled [`contribution welcome`](https://github.com/vuejs/vue-cli/issues?q=is%3Aissue+is%3Aopen+label%3A%22contribution+welcome%22) or [`help wanted`](https://github.com/vuejs/vue-cli/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22) are good issues to submit a PR for.
+* Issues labeled [`good first issue`](https://github.com/vuejs/vue-cli/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22) are great candidates to pick up if you are in the code for the first time.
+* If you are contributing significant changes, please discuss with the assignee of the issue first before starting to work on the issue.

_Sidebar.md

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 **Contributing**
 * [Contributor Code of Conduct](https://github.com/vuejs/vue/wiki/Contributor-Code-of-Conduct)
+* [[How to Contribute]]
 
 **Documentation**
 * [Guide](https://cli.vuejs.org/guide/)