Update readme.

README.md

@@ -136,6 +136,7 @@ Major Features
 - No external dependencies except for the C standard library and backend libraries.
 - Written in C and compilable as C++, enabling miniaudio to work on almost all compilers.
 - Supports all major desktop and mobile platforms, with multiple backends for maximum compatibility.
+- Supports custom backends.
 - Supports playback, capture, full-duplex and loopback (WASAPI only).
 - Device enumeration for connecting to specific devices, not just defaults.
 - Connect to multiple devices at once.