document pyright test better (#5362)

README.md

@@ -173,7 +173,12 @@ This test works similarly to `mypy_test.py`, except it uses `pytype`.
 
 ### pyright\_test.py
 
-This test requires Node.js to be installed.
+This test requires Node.js to be installed. It is currently not part of the CI,
+but it uses the same pyright version and configuration as the CI.
+```
+(.venv3)$ python3 tests/pytype_test.py                 # Check all files
+(.venv3)$ python3 tests/pytype_test.py stdlib/sys.pyi  # Check one file
+```
 
 ### mypy\_test\_suite.py