specify md readme

Author: ahebrank

setup.py

@@ -10,6 +10,7 @@
     description='Webhook Handler for GitLab',
     license='Apache License, Version 2.0',
     long_description=open('README.md', encoding='utf-8').read(),
+    long_description_content_type='text/markdown',
     install_requires=[
         'Flask>=1.0',
         'requests>=2.19.0'