Add credits for the music used in the 2D platformer demos (#461)

* Add credits for the music used in the 2D platformer demos

* Update 2d/platformer/README.md

Co-Authored-By: Hugo Locurcio <[email protected]>

* Update 2d/physics_platformer/README.md

Co-Authored-By: Hugo Locurcio <[email protected]>

Co-authored-by: Hugo Locurcio <[email protected]>

2d/physics_platformer/README.md

@@ -26,3 +26,7 @@ the physics engine, inside of `_integrate_forces()`.
 ![Screenshot of the beginning](screenshots/beginning.png)
 
 ![Screenshot of the seesaw and the player riding an enemy](screenshots/seesaw-riding.png)
+
+## Music
+
+"Pompy" by Hubert Lamontagne (madbr) https://soundcloud.com/madbr/pompy

2d/platformer/README.md

@@ -39,3 +39,7 @@ Renderer: GLES 3 (particles are not available in GLES 2)
 ![Player shooting in the direction of an enemy](screenshots/shoot.png)
 
 ![The entire level layout viewed in the editor](screenshots/layout.png)
+
+## Music
+
+"Pompy" by Hubert Lamontagne (madbr) https://soundcloud.com/madbr/pompy