Update README.rst

Author: kislyuk

README.rst

@@ -40,6 +40,9 @@ follows:
     args = parser.parse_args()
     ...
 
+If using `pyproject.toml` `project.scripts` entry points, the `PYTHON_ARGCOMPLETE_OK` should appear at the beginning of the
+file that contains the entry point.
+
 Register your Python application with your shell's completion framework by running ``register-python-argcomplete``::
 
     eval "$(register-python-argcomplete my-python-app)"