docs(readme): add Japanese README and language switch links

Author: SsuJojo

README.md

@@ -24,7 +24,8 @@
 
   <p>
     <strong>简体中文</strong> |
-    <a href="./README_EN.md">English</a>
+    <a href="./README_EN.md">English</a> |
+    <a href="./README_JA.md">日本語</a>
   </p>
 
   <br>

README_EN.md

@@ -24,7 +24,8 @@
 
   <p>
     <a href="./README.md">简体中文</a> |
-    <strong>English</strong>
+    <strong>English</strong> |
+    <a href="./README_JA.md">日本語</a>
   </p>
 
   <br>