doc: shadow described

README.md

@@ -65,9 +65,9 @@ The following tags are supported:
 | `required`    | Value   | Flag is required.                          |
 | `command`     | Command | Flag is a command.                         |
 | `call`        | Command | Function to call when the command is used. |
+| `shadow`      | Value | Copy the value to the shadow structure.    |
 | `ignore`      | -/-     | Property is ignored.                       |
 
-
 ### Callbacks
 
 The functions are called up with a receiver. The receiver is the
@@ -143,6 +143,30 @@ The function `Execute()` executes the command. See the section
 setting.Execute()
 ```
 
+### Shadow
+
+The shadow structure is used to copy the values of the flags to the
+shadow structure. The shadow structure is set using the `SetShadow()`
+and configured using the tag `shadow`.
+
+```go
+type Shadow struct {
+  Verbose bool
+  Serve   struct {
+    Host string
+    Port int
+  }
+}
+
+func main() {
+  setting := New(os.Args[0], Definition{})
+  setting.SetShadow(Shadow{})
+  setting.Parse(os.Args[1:])
+  setting.Execute()
+  fmt.Printf("Shadow: %+v", setting.GetShadow())
+}
+```
+
 ## Contributing
 
 Merge requests are welcome. For major changes, please open an issue first to discuss what