Update setup.py to use text/markdown for README

setup.py

@@ -159,6 +159,7 @@
         "Python-based version of the Barbieri/Van Hove phase shift calculation package for LEED/XPD modelling"
     ),
     long_description=(None if not os.path.exists(README) else open(README).read()),
+    long_description_content_type="text/markdown",
     classifiers=[
         "Development Status :: 4 - Beta",
         "Environment :: Console",