Fix long description of setup.py to match pypi conditions

Author: vfdev-5

setup.py

@@ -27,6 +27,7 @@ def find_version(*file_paths):
     version=version,
     description=u"Minimalistic set of image reader agnostic tools to easily iterate over large images",
     long_description=long_description,
+    long_description_content_type="text/markdown",
     author="vfdev-5",
     author_email="vfdev.5@gmail.com",
     url="https://github.com/vfdev-5/ImageTilingUtils",