Add build instructions

BUILDING.md

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+# Building Guide
+
+This guide will help you build the project on your local machine. The process will require you to provide two items:
+- A decompressed ROM of the US version of the game.
+- An elf file created from [this commit](https://github.com/zeldaret/mm/tree/23beee0717364de43ca9a82957cc910cf818de90) of the Majora's Mask decompilation.
+
+We'll walk you through acquiring these, running the required processes and finally building the project.
+
+### 1. Clone the Zelda64Recomp Repository
+This project makes use of submodules so you will need to clone the repository with the `--recurse-submodules` flag.
+
+```bash
+git clone --recurse-submodules
+# if you forgot to clone with --recurse-submodules
+# cd /path/to/cloned/repo && git submodule update --init --recursive
+```
+
+### 2. Install Dependencies
+
+You will need a few dependencies. For Linux we provide the instructions for Ubuntu, but you can find the equivalent packages for your preferred distro.
+
+```bash
+# For Ubuntu, simply run:
+cmake ninja libsdl2-dev libgtk-3-dev lld llvm clang-15
+
+# For Windows, you can use Chocolatey:
+choco cmake install ninja sdl2 gtk3 llvm
+```
+
+### 3. Creating the ELF file & decompressed ROM
+You will need to build [this commit](https://github.com/zeldaret/mm/tree/23beee0717364de43ca9a82957cc910cf818de90) of the Majora's Mask decompilation. Follow their build instructions to generate the ELF file and decompressed ROM. While building you may get the following build error:
+```bash
+RuntimeError: 'jr' instruction does not have an 'jump label' field
+```
+
+To fix this you will have to modify the problematic file `tools/disasm/disasm.py` at line 1115. Replace the line:
+```diff
+- elif insn.isJump():
++ elif insn.isJumpWithAddress():
+```
+
+Upon successful build it will generate the two required files. Copy them to the root of the Zelda64Recomp repository:
+- `mm.us.rev1.rom_uncompressed.elf`
+- `mm.us.rev1.rom_uncompressed.z64`
+
+### 4. Generating the C code
+
+Now that you have the required files, we have to build [N64Recomp](https://github.com/Mr-Wiseguy/N64Recomp) and run it to generate the C code to be compiled. The building instructions can be found in that repo. That will build the executables: `N64Recomp` and `RSPRecomp` which you should copy to the root of the Zelda64Recomp repository.
+
+After that, go back to the repository root, and run the following commands:
+```bash
+./N64Recomp[.exe] us.rev1.toml
+./RSPRecomp[.exe] aspMain.us.rev1.toml
+./RSPRecomp[.exe] njpgdspMain.us.rev1.toml
+```
+
+### 5. Building the Project
+
+Finally, you can build the project! We'll use be using CMake:
+```bash
+cmake -H. -Bbuild-cmake -GNinja -DCMAKE_BUILD_TYPE=Release # or Debug if you want to debug
+cmake --build build-cmake --target Zelda64Recompiled -j$(nproc) --config Release # or Debug
+```
+
+### 6. Profit
+
+Voilà! You should now have a `Zelda64Recompiled[.exe]` file in the `build-cmake` directory!
+
+_Note: In the game itself, you should be using a standard ROM, not the decompressed one._