Added Django version trove classifier to reusable apps tutorial.

docs/intro/reusable-apps.txt

@@ -215,6 +215,7 @@ this. For a small app like polls, this process isn't too difficult.
            classifiers=[
                'Environment :: Web Environment',
                'Framework :: Django',
+               'Framework :: Django :: X.Y',  # replace "X.Y" as appropriate
                'Intended Audience :: Developers',
                'License :: OSI Approved :: BSD License',  # example license
                'Operating System :: OS Independent',