Start section on controls

book/src/SUMMARY.md

@@ -11,3 +11,4 @@
 - [Learn agb part I - pong](./pong/01_introduction.md)
   - [The Gba struct](./pong/02_the_gba_struct.md)
   - [Sprites](./pong/03_sprites.md)
+  - [Controls](./pong/04_controls.md)

book/src/pong/04_controls.md

@@ -0,0 +1,23 @@
+# Controls
+
+In this section, we'll make the ball that we displayed in the last section move by pressing the D-Pad.
+
+# The GBA controls
+
+The GBA has 10 buttons we can read the state of, and this is the only way a player can directly control the game.
+They are the 4 directions on the D-Pad, A, B, Start, Select, and the L and R triggers.
+
+# Reading the button state
+
+There are two ways to capture the button state in **agb**, interrupts and polling.
+In most games, you will want to use polling, so that is what we will use now.
+Interrupts will be covered in a later chapter.
+
+To add button control to our game, we will need a [ButtonController](https://docs.rs/agb/latest/agb/input/struct.ButtonController.html).
+Add this near the top of your main function:
+
+```rust
+    let mut input = agb::input::ButtonController::new();
+```
+
+The button controller is not part of the `Gba` struct because it only allows for reading and not writing so does not need to be controlled by the borrow checker.