clarify preconditions for using tests.

Author: rouilj

doc/installation.txt

@@ -1734,9 +1734,12 @@ Problems? Testing your Python...
    for PostgreSQL you want to call the ``createuser`` command with the
    ``-d`` option to allow database creation.
 
-Once you've unpacked roundup's source, if you have pytest installed,
-run ``python -m pytest test`` in the source directory and make sure
-there are no errors. If there are errors, please let us know!
+This can only be done if you downloaded and unpacked the source
+distrbution. It will not work if you used `pip install` as the test
+suite is not installed. Once you've unpacked roundup's source, if you
+have pytest installed, run ``python -m pytest test`` in the source
+directory and make sure there are no errors. If there are errors,
+please let us know!
 
 .. _`user guide`: user_guide.html
 .. _`roundup specification`: spec.html