doc: pytest: mention additional requirements for venv

Add a paragraph explaining that in addition to the requirements.txt
for test/py/test.py itself, users may need to install additional python
packages depending on the U-Boot configuration being built.

Reviewed-by: Simon Glass <simon.glass@canonical.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@kernel.org> # sandbox
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>

doc/develop/pytest/usage.rst

@@ -90,6 +90,10 @@ environment.  This can be done via the following commands:
     $ . ./venv/bin/activate
     $ pip install -r test/py/requirements.txt
 
+In order to use the ``--build`` option of ``test.py`` there may be additional
+python packages required depending on the configuration being built. These can
+be found with ``find . -name "*.requirements*.txt"``.
+
 Testing sandbox
 ---------------