add maintainability

README.md

@@ -5,6 +5,7 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/viney-shih/go-cache)](https://goreportcard.com/report/github.com/viney-shih/go-cache)
 [![codecov](https://codecov.io/gh/viney-shih/go-cache/branch/master/graph/badge.svg?token=QKRiNSU5Gn)](https://codecov.io/gh/viney-shih/go-cache)
 [![Coverage Status](https://coveralls.io/repos/github/viney-shih/go-cache/badge.svg?branch=master)](https://coveralls.io/github/viney-shih/go-cache?branch=master)
+[![Maintainability](https://api.codeclimate.com/v1/badges/5b70576957b8af88de6e/maintainability)](https://codeclimate.com/github/viney-shih/go-cache/maintainability)
 [![License](http:https://img.shields.io/badge/License-Apache_2-red.svg?style=flat)](http:https://www.apache.org/licenses/LICENSE-2.0)
 
 <p align="center">