Suggest Markdown mimetype to pipy for long_description

Author: einarf

setup.py

@@ -10,6 +10,7 @@
     url='https://github.com/greenmoss/PyWavefront',
     description='Python/pyglet library for importing Wavefront .obj files',
     long_description=open('README.md').read(),
+    long_description_content_type='text/markdown',
     license='BSD',
     classifiers=[
         'Development Status :: 4 - Beta',