Update README

Author: hisasann

README.md

@@ -49,6 +49,7 @@ For detailed explanation on how things work, checkout [Nuxt.js docs](https://nux
 - [search/_condition](https://typescript-nuxtjs-boilerplate.netlify.com/example/search/?hoge=foo&bar=fuga)
 - [img-lazy-load](https://typescript-nuxtjs-boilerplate.netlify.com/example/img-lazy-load)
 - [type-safe-vuex](https://typescript-nuxtjs-boilerplate.netlify.com/example/type-safe-vuex)
+- [extend-css-class](https://typescript-nuxtjs-boilerplate.netlify.com/example/extend-css-class)
 
 ## Maintainers