Update README.md

2018-06-17 17:02:01 +09:00 · 2018-06-17 17:02:01 +09:00 · f941443ff0
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 [![Build Status](https://travis-ci.org/google/sentencepiece.svg?branch=master)](https://travis-ci.org/google/sentencepiece) 
 [![Coverage Status](https://coveralls.io/repos/github/google/sentencepiece/badge.svg?branch=master)](https://coveralls.io/github/google/sentencepiece?branch=master)
+[![GitHub Issues](https://img.shields.io/github/issues/google/sentencepiece.svg)](https://github.com/google/sentencepiece/issues)
 [![PyPI version](https://badge.fury.io/py/sentencepiece.svg)](https://badge.fury.io/py/sentencepiece)
 [![Contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg)](CONTRIBUTING.md)
 [![License](https://img.shields.io/badge/License-Apache%202.0-brightgreen.svg)](https://opensource.org/licenses/Apache-2.0)