add national flag

README.md

@@ -1,4 +1,4 @@
-- [日本語](./README_JP.md)
+- [日本語 🇯🇵](./README_JP.md)
 
 <p align="center">
   <a href="https://growi.org">

README_JP.md

@@ -1,4 +1,4 @@
-- [English](./README.md)
+- [English 🇺🇸](./README.md)
   <p align="center">
     <a href="https://growi.org">
       <img src="https://user-images.githubusercontent.com/1638767/38254268-d4476bbe-3793-11e8-964c-8865d690baff.png" width="240px">