Add a prompt: per-app proxy requires NAT mode under Android 4.x

src/main/res/values-v21/strings.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="proxied_apps_summary">Set proxy for selected apps</string>
+</resources>

src/main/res/values-zh-v21/strings.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="proxied_apps_summary">为应用程序分别设置代理</string>
+</resources>

src/main/res/values-zh/strings.xml

@@ -26,7 +26,7 @@
   <string name="auto_set_gfwlist_summary">绕过所有位于中国的网站</string>
   <string name="route_list">路由</string>
   <string name="proxied_apps">分应用代理</string>
-  <string name="proxied_apps_summary">为应用程序分别设置代理</string>
+  <string name="proxied_apps_summary">为应用程序分别设置代理，在 4.x 下需要启用 NAT 模式</string>
   <string name="on">开</string>
   <string name="bypass_apps">绕行模式</string>
   <string name="bypass_apps_summary">启用该选项，以使所选应用程序的流量不经过代理</string>

src/main/res/values/strings.xml

@@ -54,7 +54,7 @@
   </string>
   <string name="route_list">Route</string>
   <string name="proxied_apps">Per-App Proxy</string>
-  <string name="proxied_apps_summary">Set proxy for selected apps</string>
+  <string name="proxied_apps_summary">Set proxy for selected apps, requires NAT mode under Android 4.x</string>
   <string name="on">On</string>
   <string name="bypass_apps">Bypass Mode</string>
   <string name="bypass_apps_summary">Enable this option to bypass selected apps</string>