chore(readm): add gitter

Author: azu

README.md

@@ -1,4 +1,5 @@
-# textlint-rule-ja-no-mixed-period [![Build Status](https://travis-ci.org/textlint-ja/textlint-rule-ja-no-mixed-period.svg?branch=master)](https://travis-ci.org/textlint-ja/textlint-rule-ja-no-mixed-period)
+# textlint-rule-ja-no-mixed-period [![Build Status](https://travis-ci.org/textlint-ja/textlint-rule-ja-no-mixed-period.svg?branch=master)](https://travis-ci.org/textlint-ja/textlint-rule-ja-no-mixed-period) [![Gitter](https://badges.gitter.im/textlint-ja/textlint-ja.svg)](https://gitter.im/textlint-ja/textlint-ja?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+
 
 文末の句点(。)の統一 OR 抜けをチェックするtextlintルール