docs: Moved CREDITS and INSTALL to markdown format.

CREDITS.md

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+# Simple DirectMedia Layer CREDITS
+
+Thanks to everyone who made this possible, including:
+
+- Cliff Matthews, for giving me a reason to start this project. :)  -- Executor rocks!  *grin*
+- Ryan Gordon for helping everybody out and keeping the dream alive. :)
+- Gabriel Jacobo for his work on the Android port and generally helping out all around.
+- Philipp Wiesemann for his attention to detail reviewing the entire SDL code base and proposes patches.
+- Andreas Schiffler for his dedication to unit tests, Visual Studio projects, and managing the Google Summer of Code.
+- Mike Sartain for incorporating SDL into Team Fortress 2 and cheering me on at Valve.
+- Alfred Reynolds for the game controller API and general (in)sanity
+- Jørgen Tjernø¸ for numerous magical macOS fixes.
+- Pierre-Loup Griffais for his deep knowledge of OpenGL drivers.
+- Julian Winter for the SDL 2.0 website.
+- Sheena Smith for many months of great work on the SDL wiki creating the API documentation and style guides.
+- Paul Hunkin for his port of SDL to Android during the Google Summer of Code 2010.
+- Eli Gottlieb for his work on shaped windows during the Google Summer of Code 2010.
+- Jim Grandpre for his work on multi-touch and gesture recognition during
+  the Google Summer of Code 2010.
+- Edgar "bobbens" Simo for his force feedback API development during the
+  Google Summer of Code 2008.
+- Aaron Wishnick for his work on audio resampling and pitch shifting during
+  the Google Summer of Code 2008.
+- Holmes Futrell for his port of SDL to the iPhone and iPod Touch during the
+  Google Summer of Code 2008.
+- Jon Atkins for SDL_image, SDL_mixer and SDL_net documentation.
+- Everybody at Loki Software, Inc. for their great contributions!
+
+ And a big hand to everyone else who has contributed over the years.
+
+THANKS! :)
+
+    -- Sam Lantinga <slouken@libsdl.org>
+

INSTALL.md

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+# To compile and install SDL:
+
+##  Windows with Visual Studio:
+
+Read ./docs/README-visualc.md
+
+## Windows building with mingw-w64 for x86:
+
+Run: `cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=build-scripts/cmake-toolchain-mingw64-i686.cmake && cmake --build build && cmake --install build`
+
+## Windows building with mingw-w64 for x64:
+
+Run: `cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=build-scripts/cmake-toolchain-mingw64-x86_64.cmake && cmake --build build && cmake --install build`
+
+## macOS with Xcode:
+
+Read docs/README-macos.md
+
+## macOS from the command line:
+
+Run: `cmake -S . -B build && cmake --build build && cmake --install build`
+
+## Linux and other UNIX systems:
+
+Run: `cmake -S . -B build && cmake --build build && cmake --install build`
+
+## Android:
+
+Read docs/README-android.md
+
+## iOS:
+
+Read docs/README-ios.md
+
+## Using CMake:
+
+Read docs/README-cmake.md
+
+# Example code
+
+Look at the example programs in ./test, and check out the online
+documentation at https://wiki.libsdl.org/SDL3/
+
+# Discussion
+
+## Forums/mailing lists
+
+Join the SDL developer discussions, sign up on
+
+https://discourse.libsdl.org/
+
+and go to the development forum
+
+https://discourse.libsdl.org/c/sdl-development/6
+
+Once you sign up, you can use the forum through the website, or as a mailing
+list from your email client.
+
+## Announcement list
+
+Sign up for the announcement list through the web interface:
+
+https://www.libsdl.org/mailing-list.php
+