Create CONTRIBUTING.md

CONTRIBUTING.md

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+## Contributing
+
+### Dependencies
+
+There are a few additional dependencies needed when contributing to this project. You can install them with `./install.sh --extra`.
+
+### WSL
+
+We provide [windows_terminal.bat](tools/windows_terminal.bat) to open a [Windows Terminal](https://www.microsoft.com/en-us/p/windows-terminal/9n0dx20hk701) with a recommended layout. Make sure a default distro (e.g. `wsl --set-default ubuntu`) is set beforehand.
+
+### Rebuilding
+
+If you use Visual Studio Code, you can use _Run Build Task_ (Ctrl+Shift+B) to run `make`. Any errors or warnings generated by the compiler will show up in the _Problems_ tab.
+
+### Matching a function
+
+#### Setup
+
+Once you've created a successful (`OK`) build, copy `build/` to `expected/build/`:
+
+```sh
+$ mkdir -p expected
+$ cp -r build expected
+```
+
+#### Roughly converting assembly to C
+
+Decide on a function to match. These can be found in the subdirectories of `asm/nonmatchings/`. Currently, functions which use float constants, data sections, or jump tables are unmatchable.
+
+Take the relevant `.s` file and pass it to [mips_to_c](https://github.com/matt-kempster/mips_to_c) ([web version](https://simonsoftware.se/other/mips_to_c.py)).
+
+You can also use mips_to_c locally installed to a destination of your choice. Then register a function in `~/.bashrc` that calls `path/to/mips_to_c.py (with args)`:
+```
+sudo apt update
+sudo apt install python3-pip
+sudo git clone https://github.com/matt-kempster/mips_to_c.git path/to/mips_to_c
+```
+
+Here's a starter function you can use:
+```sh
+# don't forget to replace /path/to/mips_to_c with your path
+function mipstoc() {
+    if [ "$#" -gt 1 ]; then
+        /path/to/mips_to_c/mips_to_c.py $@;
+    else
+        printf "Please call mipstoc using this format and make sure you're at the repo root:";
+        printf "\nmipstoc \033[0;31marg1 - the nonmatching asm file\033[0m \033[0;34marg2 - the target function\033[0m \033[0;33margN - any of the optional mips_to_c.py flags\033[0m";
+        printf "\nmipstoc \033[0;31m./asm/nonmatchings/code_13870_len_6980/func_8003B3D0.s\033[0m \033[0;34mfunc_8003B3D0\033[0m \033[0;33m--flag1 --flag2 --flagN\033[0m\n";
+        /path/to/mips_to_c/mips_to_c.py;
+    fi
+}
+export -f mipstoc
+```
+
+Open up the relevant `.c` file and replace the function's `INCLUDE_ASM` macro with the output from mips_to_c. Run the following command to attempt to compile, replacing `function_name` with the name of the function you're working with:
+
+```sh
+./diff.py -mwo function_name
+```
+
+Fix any errors and rerun `diff.py`. This will involve typing the function signature correctly, which you will probably find in [Star Rod's library database](https://github.com/nanaian/star-rod/blob/master/database/common_func_library.lib). See also [common_structs.h](include/common_structs.h).
+
+Once a successful build is made, `diff.py` will show you the difference between the original game's assembly (on the left) and what your C code generated (on the right).
+
+#### Matching the function
+
+You're on your own now. Get your C code compiling to match the original assembly! `diff.py`, when running, will automatically recompile your code whenever you save the `.c` file.
+
+If you use Visual Studio Code, you can use _Run Test Task_ to run `diff.py` and show you errors and warnings from the compiler inline. You might want to attach _Run Test Task_ to a keybinding, as you'll be using it often.
+
+#### After matching
+
+Once you've matched a function, run the following scripts:
+
+```sh
+$ ./coverage.py --delete-matched
+$ ./format.sh
+```
+
+If `format.sh` has any problems with your code, go and fix the issues. If you can't fix a warning without making the function not match anymore, append `// NOLINT` to the offending line.