Write the introduction for the pong game

book/src/SUMMARY.md

@@ -8,3 +8,4 @@
         - [Windows setup]()
         - [Mac OS setup]()
     - [Building the game](./setup/building.md)
+- [Learn agb part I - pong](./pong/introduction.md)

book/src/pong/introduction.md

@@ -0,0 +1,14 @@
+# Learn agb part I - pong
+
+In this section, we'll make a simple pong style game for the Game Boy Advance using `agb`.
+You will learn:
+
+* How to import graphics using `agb`.
+* What Game Boy Advance sprites are and how to put them on the screen.
+* How to detect button input and react to it.
+* How to add a static background.
+* How to make a dynamic background for a score display.
+* How to add music to your game.
+* How to add sound effects to your game.
+
+With this knowledge, you'll be well equipped to start making your own games!