Guide/Troubleshooting for vendor/build?

まだ誰も着手していません。

After digging through the source for a while and trying to see if httptools might meet my needs I decided it was time to add a test case to `tests/test_parser.py`

I was attempting to follow the Development instructions https://github.com/MagicStack/httptools?tab=readme-ov-file#development and got an error during `pip install -e .[test]`.

```
httptools/parser/parser.c:1260:10: fatal error: llhttp.h: No such file or directory
1260 | #include "llhttp.h"
| ^~~~~~~~~~
compilation terminated.
```

Ubuntu doesn't have a `node-llhttp-dev` package and `node-llhttp` doesn't help; that's about where my C build knowledge ends sadly.

I was so confused until I found the `vendor` directory with some anemic submodules; apparently the `--recursive` in the `git clone` step is kind of important 😉 In my case the alternative solution was `git submodule update --init` after the fact; whoops!

Then I remembered how desperately modern python's are trying to get rid of setuptools:

```
(.venv) clayg@ThinkStation:~/Workspace/scratch/httptools$ make
python3 setup.py build_ext --inplace
Traceback (most recent call last):
File "/mnt/c/Users/cgerrard/Workspace/scratch/httptools/setup.py", line 10, in
from setuptools import setup, Extension
ModuleNotFoundError: No module named 'setuptools'
make: *** [Makefile:9: compile] Error 1
```

... but I eventually got to write my test (surprise: llhttp does NOT allow for non-ascii bytes in header names or values: https://gist.github.com/clayg/174d8e427c4263230b3cdf6591f3365f)

Anyway; would you find it helpful to include some expanded notes/guide/troubleshooting on the build requirements/process (or how to update the vendored submodules?) added to the existing README development section or would it be better to keep that as simple as possible and make a new/different file?

README の開発セクションから始め、setup.py と Makefile のコマンドと比較します。報告されている `pip install -e .[test]` と `make` のパスを確認し、必要な build 手順、再帰的な vendor サブモジュール、setuptools の要件を文書化します。新しいコントリビューターが報告されたエラーに遭遇せずに開発環境のセットアップに従える状態になれば完了です。

索引モデルが issue の本文から書いたものです。