SDL: Update migration guide with info on trackballs

From e8c16ce48331525ee3f653fc75ca8b322be7bc30 Mon Sep 17 00:00:00 2001
From: Eric <[EMAIL REDACTED]>
Date: Fri, 28 Apr 2023 21:51:04 -0400
Subject: [PATCH] Update migration guide with info on trackballs

---
 docs/README-migration.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/README-migration.md b/docs/README-migration.md
index 4448f6f1e409..31d1ff8518f1 100644
--- a/docs/README-migration.md
+++ b/docs/README-migration.md
@@ -476,6 +476,7 @@ The following functions have been removed:
 * SDL_JoystickGetDeviceType() - replaced with SDL_GetJoystickInstanceType()
 * SDL_JoystickGetDeviceVendor() - replaced with SDL_GetJoystickInstanceVendor()
 * SDL_JoystickNameForIndex() - replaced with SDL_GetJoystickInstanceName()
+* SDL_JoystickNumBalls() - API has been removed, see https://github.com/libsdl-org/SDL/issues/6766
 * SDL_JoystickPathForIndex() - replaced with SDL_GetJoystickInstancePath()
 * SDL_NumJoysticks() - replaced with SDL_GetJoysticks()