add the "tree-shaking" badge

Author: gfx

README.md

@@ -1,6 +1,6 @@
 # MessagePack for JavaScript/ECMA-262
 
-[![npm version](https://img.shields.io/npm/v/@msgpack/msgpack.svg)](https://www.npmjs.com/package/@msgpack/msgpack) ![CI](https://github.com/msgpack/msgpack-javascript/workflows/CI/badge.svg) [![codecov](https://codecov.io/gh/msgpack/msgpack-javascript/branch/master/graphs/badge.svg)](https://codecov.io/gh/msgpack/msgpack-javascript) [![bundlephobia](https://badgen.net/bundlephobia/minzip/@msgpack/msgpack)](https://bundlephobia.com/result?p=@msgpack/msgpack)
+[![npm version](https://img.shields.io/npm/v/@msgpack/msgpack.svg)](https://www.npmjs.com/package/@msgpack/msgpack) ![CI](https://github.com/msgpack/msgpack-javascript/workflows/CI/badge.svg) [![codecov](https://codecov.io/gh/msgpack/msgpack-javascript/branch/master/graphs/badge.svg)](https://codecov.io/gh/msgpack/msgpack-javascript) [![minzip](https://badgen.net/bundlephobia/minzip/@msgpack/msgpack)](https://bundlephobia.com/result?p=@msgpack/msgpack) [![tree-shaking](https://badgen.net/bundlephobia/tree-shaking/@msgpack/msgpack)](https://bundlephobia.com/result?p=@msgpack/msgpack)
 
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