add system update api

docs/README.md

@@ -0,0 +1,3 @@
+# DOCUMENT
+
+[Here](http://docs.onlinejudge.me/)

docs/data.json

@@ -0,0 +1,16 @@
+{
+  "update": [
+    {
+      "version": "2017-12-25",
+      "level": 1,
+      "title": "Update at 2017-12-25",
+      "details": [
+        "Fix some issues under IE/Edge",
+        "Add backend error reporter",
+        "New email template",
+        "A more flexible throttling function",
+        "Other bugs and enhancements"
+      ]
+    }
+  ]
+}