extra docs for running scripts (#373)

* extra docs for running scripts

docs/source/customize.rst

@@ -216,3 +216,12 @@ configured to use a specific kernel for each language::
 
 In this case it will use the `xeus-python
 <https://github.com/QuantStack/xeus-python/>`_. kernel to run `.py` files.
+
+Note that the script will be executed as notebook with a single cell, meaning
+that only the last expression will be printed as output. Use the Jupyter
+display mechanism to output any text or rich output such as Jupyter widgets. For
+Python this would be a call to `IPython.display.display`.
+
+Using `Jupytext <https://github.com/mwouts/jupytext>`_ is another way to support
+script files. After installing jupytext, voila will see script files as if they
+are notebooks, and requires no extra configuration.