Up-to-date QNX support (#14806)

Co-authored-by: Roberto Speranza <rsperanza@qnx.com>
Co-authored-by: Darcy Phipps <dphipps@qnx.com>
Co-authored-by: Pierce McKinnon <pimckinnon@qnx.com>

docs/README-qnx.md

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+QNX
+=======
+
+SDL port for QNX, providing both screen and Wayland video backends.
+
+This was originally contributed by Elad Lahav for QNX 7.0.
+
+The port was later improved and adapted for QNX 8.0 by:
+- Ethan Leir
+- Roberto Speranza
+- Darcy Phipps
+- Jai Moraes
+- Pierce McKinnon
+
+Further changes to enable Wayland with the EGL backend were made by Felix Xing
+and Aaron Bassett.
+
+
+## Building
+
+Building SDL3 for QNX requires Wayland to be built and installed. The commands
+to build it are,
+```bash
+# Note, if you're cross-compiling, you will need to source qnxsdp-env.sh and
+# provide the path to a cmake toolchain file with -DCMAKE_TOOLCHAIN_FILE=$TOOLCHAIN_DIR/qnx.nto.toolchain.cmake
+cmake -B build -DCMAKE_BUILD_TYPE=Release -DSDL_X11=0
+cmake --build build
+cmake --install build
+```
+
+## QNX self-hosted
+
+QNX provides a self-hosted environment, available with [the free license](https://www.qnx.com/products/everywhere/).
+This is the easiest way to get your hands on SDL.
+
+## QNX build-files
+
+You can find the cross-compiled build tools at https://github.com/qnx-ports/build-files
+
+## Notes - screen
+
+- Currently, only software and OpenGLES2 rendering is supported.
+- Unless your application is managed by a window manager capable of closing the application, you will need to quit it yourself.
+- Restraining the mouse to a window or warping the mouse cursor will not work.