note that xonsh requires pip 24 for an editable install and development (#5460)

with pip 22, i was getting extremely strange errors like this one:
```
$ pip install '.[dev]'
Processing /home/jyn/src/xonsh
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
WARNING: unknown 0.0.0 does not provide the extra 'dev'
Building wheels for collected packages: UNKNOWN
  Building wheel for UNKNOWN (pyproject.toml) ... done

Successfully installed UNKNOWN-0.0.0
$ pytest
tests/test_lib/test_subprocess.xsh:10: in <module>
    from xonsh.pytest.tools import ON_WINDOWS
E   ModuleNotFoundError: No module named 'xonsh.pytest'
```

see
https://xonsh.zulipchat.com/#narrow/stream/435069-xonsh-dev/topic/install.20xonsh.20for.20development.20without.20sudo.20rights
for more context.

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## For community
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CONTRIBUTING.rst

@@ -19,6 +19,8 @@ develop xonsh.
 Making Your First Change
 ========================
 
+.. note:: xonsh requires at least `pip 24.0` for `pyproject.toml` support. You can upgrade old versions using `pip install -U pip`.
+
 First, install xonsh from source and open a xonsh shell in your favorite
 terminal application. See installation instructions for details, but it
 is recommended to do an 'editable' install via `pip'