Document Header widget

docs/widgets/header.md

@@ -1 +1,34 @@
 # Header
+
+## Description
+
+A simple header widget which docks itself to the top of the parent container.
+
+## Example
+
+The example below shows an app with a `Header`.
+
+=== "Output"
+
+    ```{.textual path="docs/examples/widgets/header.py"}
+    ```
+
+=== "header.py"
+
+    ```python
+    --8<-- "docs/examples/widgets/header.py"
+    ```
+
+## Reactive Attributes
+
+| Name   | Type   | Default | Description                                                                                                                                                                                      |
+|--------|--------|---------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `tall` | `bool` | `True`  | Whether the `Header` widget is displayed as tall or not. The tall variant is 3 cells tall by default. The non-tall variant is a single cell tall. This can be toggled by clicking on the header. |
+
+## Messages
+
+This widget sends no messages.
+
+## See Also
+
+* [Header](../reference/header.md) code reference