SDL_mixer: Expand CMake line a bit in README.txt

From 5469c8642fd4478715e4f68ea2e3ddc421682ea8 Mon Sep 17 00:00:00 2001
From: Anonymous Maarten <[EMAIL REDACTED]>
Date: Wed, 27 Jul 2022 16:28:00 +0200
Subject: [PATCH] Expand CMake line a bit in README.txt

---
 README.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.txt b/README.txt
index c5187d02..2b9cca85 100644
--- a/README.txt
+++ b/README.txt
@@ -22,7 +22,7 @@ This library is under the zlib license, see the file "LICENSE.txt" for details.
 
 Note:
 Support for software MIDI, MOD, and Opus are not included by default because of the size of the decode libraries, but you can get them by running external/download.sh
-- When building with CMake, you can enable the appropriate SUPPORT_* options defined in CMakeLists.txt.
+- When building with CMake, you can enable the appropriate SDL2MIXER_* options defined in CMakeLists.txt. SDL2MIXER_VENDORED allows switching between system and vendored libraries.
 - When building with configure/make, you can build and install them normally and the configure script will detect and use them.
 - When building with Visual Studio, you will need to build the libraries and then add the appropriate LOAD_* preprocessor define to the Visual Studio project.
 - When building with Xcode, you can edit the config at the top of the project to enable them, and you will need to include the appropriate framework in your application.