Make twine happy by specifying content type

- Seems pypi requires the content type of the long description to
  be explicit. Set it to markdown.

Author: alexmgr

setup.py

@@ -23,6 +23,7 @@ def read_(file_name):
     url="https://github.com/alexmgr/tinyec",
     download_url="https://github.com/alexmgr/tinyec/archive/v0.4.0.tar.gz",
     long_description=read_("README.md"),
+    long_description_content_type="text/markdown",
     test_suite="nose.collector",
     tests_require=["nose"],
     classifiers=[