SDL: docs: Add

From 27369b3e40c0eb0267f4a54a80a211ca91b12ca0 Mon Sep 17 00:00:00 2001
From: Cameron Cawley <[EMAIL REDACTED]>
Date: Fri, 29 Oct 2021 23:28:50 +0100
Subject: [PATCH] docs: Add

 docs/ | 41 +++++++++++++++++++++++++++++++++++++++++
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 create mode 100644 docs/

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+* RISC OS 3.5 or later.
+* [SharedUnixLibrary](
+* [DigitalRenderer](, for audio support.
+* [Iconv](, for `SDL_iconv` and related functions.
+Currently, SDL2 for RISC OS only supports compiling with GCCSDK under Linux. Both the autoconf and CMake build systems are supported.
+The following commands can be used to build SDL2 for RISC OS using autoconf:
+    ./configure --host=arm-unknown-riscos --prefix=$GCCSDK_INSTALL_ENV --disable-gcc-atomics
+    make
+    make install
+The following commands can be used to build SDL2 for RISC OS using CMake:
+    cmake --build build-riscos
+    cmake --build build-riscos --target install
+Current level of implementation
+The video driver currently provides full screen video support with keyboard and mouse input. Windowed mode is not yet supported, but is planned in the future. Only software rendering is supported.
+The filesystem APIs return either Unix-style paths or RISC OS-style paths based on the value of the `__riscosify_control` symbol, as is standard for UnixLib functions.
+The audio, loadso, thread and timer APIs are currently provided by UnixLib.
+GCC atomics are currently broken on some platforms, meaning it's currently necessary to compile with `--disable-gcc-atomics` using autotools or `-DSDL_GCC_ATOMICS=OFF` using CMake.
+The joystick, locale and power APIs are not yet implemented.