chore: add badge of lerna

sorrycc · sorrycc · commit cb75c1dd4349 · 2018-12-10T11:30:58.000+08:00
diff --git a/README.md b/README.md
@@ -5,6 +5,7 @@ English | [简体中文](./README_zh-CN.md)
 [![NPM version](https://img.shields.io/npm/v/umi.svg?style=flat)](https://npmjs.org/package/umi)
 [![Build Status](https://img.shields.io/travis/umijs/umi.svg?style=flat)](https://travis-ci.org/umijs/umi)
 [![NPM downloads](http://img.shields.io/npm/dm/umi.svg?style=flat)](https://npmjs.org/package/umi)
+[![lerna](https://img.shields.io/badge/maintained%20with-lerna-cc00ff.svg)](https://lernajs.io/)
 
 🌋 Pluggable enterprise-level react application framework.
 
