setup: set long description content type for markdown

So we can use the README as is on PyPI

Author: xrmx

setup.py

@@ -42,6 +42,7 @@
     license='MIT License',
     description='A simple Django app to add JSON widget into Django Administration.',
     long_description=README,
+    long_description_content_type='text/markdown',
     url='https://github.com/abogushov/django-admin-json-editor',
     author='Alexander Bogushov',
     author_email='[email protected]',