Add a FAQ about aligning things in the middle of a container

questions/align-center-middle.question.md

@@ -0,0 +1,34 @@
+---
+title: "How do I center a widget in a screen?"
+alt_titles:
+  - "centre a widget"
+  - "center a control"
+  - "centre a control"
+---
+
+To center a widget within a container use
+[`align`](https://textual.textualize.io/styles/align/). But remember that
+`align` works on the *children* of a container, it isn't something you use
+on the child you want centered.
+
+For example, here's an app that shows a `Button` in the middle of a
+`Screen`:
+
+```python
+from textual.app import App, ComposeResult
+from textual.widgets import Button
+
+class ButtonApp(App):
+
+    CSS = """
+    Screen {
+        align: center middle;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        yield Button("PUSH ME!")
+
+if __name__ == "__main__":
+    ButtonApp().run()
+```