README: updated for SDL3_mixer.

Fixes #708.

Author: icculus

README.md

@@ -0,0 +1,23 @@
+
+# SDL_mixer 3.0
+
+The latest version of this library is available from GitHub:
+https://github.com/libsdl-org/SDL_mixer/releases
+
+SDL_mixer 3.0 ("SDL3_mixer") is an audio management library. It provides
+decoding of many popular audio file formats, mixing, various DSP processing
+effects and positional audio.
+
+Audio data can be preloaded, or streamed on-the-fly into the mixer.
+
+SDL3_mixer supports multiple audio devices, and can even mix to a memory buffer
+instead of feeding audio hardware in real time.
+
+SDL3_mixer is a complete redesign, rewritten almost from scratch to address
+deficiencies in SDL2_mixer. The API is completely different and, in our
+opinion, a significant improvement in power and ease of use. Please refer to
+docs/README-migration.md for details on how to migrate your program to the new
+API.
+
+This library is under the zlib license, see the file LICENSE.txt for details.
+