Add unit test folder (#698)

Add unit test folder to packaging tutorial. 

This at least gives people a default starting
point while we continue discussion in #619
on how to introduce the fact that this is *only*
a default, and that how best to lay out
test directories is a topic of some debate.

Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

source/tutorials/packaging-projects.rst

@@ -44,11 +44,18 @@ them in the following steps.
     packaging_tutorial/
       example_pkg/
         __init__.py
+      tests/
       setup.py
       LICENSE
       README.md
 
 
+Creating a test folder
+----------------------
+
+:file:`tests/` is a placeholder for unit test files. Leave it empty for now.
+
+
 Creating setup.py
 -----------------