Create build instructions. (#7)

As the title says, I've created (hopefully) detailed enough instruction
for building the game.

Let me know if I've missed anything!

BUILDING.md

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+# Requirements
+  * A Super Metroid rom (Make sure to rename it to `sm.smc`)
+  * libsdl2-dev
+  
+ For Linux/MacOS you must install these for your desired OS:
+ * Ubuntu/Debian: `sudo apt install libsdl2-dev`
+ * Fedora Linux: `sudo dnf in sdl2-devel`
+ * Arch Linux: `sudo pacman -S sdl2`
+ * macOS: `brew install sdl2`
+
+# Windows
+
+## Building with MSYS2
+
+Dependencies and requirements:
+
+Note: *Make sure you're using MINGW64, otherwise it won't work.*
+  * The `libsdl2-dev` library
+  * [MSYS2](https://www.msys2.org)
+
+1. Install MSYS2 on your machine.
+2. Clone the SM repository. `git clone https://github.com/snesrev/sm`
+3. Place the copy of your rom in the main directory.
+4. Install `libsdl2-dev` with `sdl2-config --cflags`
+5. After that it outputs `-IC:/msys64/mingw64/include/SDL2 -Dmain=SDL_main`, type `sdl2-config --libs`, it should output `-LC:/msys64/mingw64/lib -lmingw32 -mwindows -lSDL2main -lSDL2`
+6. Now install Make and GCC, type `pacman -S make && pacman -S mingw-w64-x86_64-gcc`
+
+NOTE: *Make sure you're in the `sm` directory.*
+
+After you've done installing everything, cd to `sm` folder. Type `make`
+In order to speed up the compilation, type `make -j16`
+
+## Building with Visual Studio
+
+Dependencies and requirements:
+ * The `libsdl2-dev` library, which is automatically installed with NuGet.
+ * [Visual Studio Community 2022](https://visualstudio.microsoft.com)
+
+Download VS installer. On installer prompt, make sure you're on "Workloads" and check `Desktop Development with C++` this will install the necessary deps for compilation.
+
+1. Open `sm.sln` solution.
+2. Change the build target from `Debug` to `Release`
+3. Build the solution.
+
+## Building with Tiny C Compiler
+
+ Dependencies and requirements:
+ * You'll need [TCC](https://github.com/FitzRoyX/tinycc/releases/download/tcc_20221020/tcc_20221020.zip) and [SDL2](https://github.com/libsdl-org/SDL/releases/download/release-2.24.1/SDL2-devel-2.24.1-VC.zip) in order to compile using TCC.
+
+1. Unzip both TCC and SDL and place them in `third_party` folder.
+2. Double click `run_with_tcc.bat`
+3. Wait for it to compile and the game will automatically boot-up.
+
+# Linux/MacOS
+
+Cd to your SM root folder and open the terminal and type:
+```sh
+make
+```
+
+For more advanced usage:
+```sh
+make -j$(nproc) # run on all core
+make clean all  # clear gen+obj and rebuild
+CC=clang make   # specify compiler
+```