📝 docs: add codecov badge

Author: kazupon

README.md

@@ -1,6 +1,7 @@
 # :globe_with_meridians: vue-i18n-locale-message
 
 [![CircleCI](https://circleci.com/gh/kazupon/vue-i18n-locale-message.svg?style=svg)](https://circleci.com/gh/kazupon/vue-i18n-locale-message)
+[![codecov](https://codecov.io/gh/kazupon/vue-i18n-locale-message/branch/master/graph/badge.svg)](https://codecov.io/gh/kazupon/vue-i18n-locale-message)
 
 i18n locale messages management tool / library for vue-i18n