Document how to run under pipx run (#1951)

README.md

@@ -34,9 +34,11 @@ The `pip-compile` command lets you compile a `requirements.txt` file from
 your dependencies, specified in either `pyproject.toml`, `setup.cfg`,
 `setup.py`, or `requirements.in`.
 
-Run it with `pip-compile` or `python -m piptools compile`. If you use
-multiple Python versions, you can also run `py -X.Y -m piptools compile` on
-Windows and `pythonX.Y -m piptools compile` on other systems.
+Run it with `pip-compile` or `python -m piptools compile` (or
+`pipx run --spec pip-tools pip-compile` if `pipx` was installed with the
+appropriate Python version). If you use multiple Python versions, you can also
+run `py -X.Y -m piptools compile` on Windows and `pythonX.Y -m piptools compile`
+on other systems.
 
 `pip-compile` should be run from the same virtual environment as your
 project so conditional dependencies that require a specific Python version,