Update setup.py

Adding a `description` field to `setup.py`

setup.py

@@ -5,6 +5,7 @@ setup(
     name='wordcloud',
     version='1.0.0',
     url='https://github.com/amueller/word_cloud',
+    description='A little word cloud generator',
     license='MIT',
     ext_modules=cythonize('wordcloud/query_integral_image.pyx'),
     packages=['wordcloud'],