🎉 Translated into Japanese completely

re-fort · re-fort · commit 0272728d1152 · 2017-08-06T18:18:35.000+09:00
diff --git a/docs/LANGS.md b/docs/LANGS.md
@@ -1 +1,2 @@
 * [English](en/)
+* [日本語](ja/)
diff --git a/docs/ja/book.json b/docs/ja/book.json
@@ -0,0 +1,3 @@
+{
+  "root": ""
+}
