docs: add lerna badge in README (#786)

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**What kind of change does this PR introduce?**
docs/readme change

**Did you add tests for your changes?**
Not required

**If relevant, did you update the documentation?**

**Summary**
webpack-cli is using [lerna](https://lernajs.io/) that's why I think we should add lerna badge.


**Does this PR introduce a breaking change?**
No

**Other information**

README.md

@@ -19,6 +19,7 @@
 [![chat on gitter](https://badges.gitter.im/webpack/webpack.svg)](https://gitter.im/webpack/webpack)
 [![Install Size](https://packagephobia.now.sh/badge?p=webpack-cli)](https://packagephobia.now.sh/result?p=webpack-cli)
 [![npm](https://img.shields.io/npm/dw/webpack-cli.svg)](https://www.npmjs.com/package/webpack-cli)
+[![lerna](https://img.shields.io/badge/maintained%20with-lerna-cc00ff.svg)](https://lernajs.io/)
 
 
 * [About](#about)