Add long_description_content_type in setup.py

When running twine check:

```
2020-10-21T15:07:30.1070752Z Checking wheelhouse/wbia_lightnet-3.0.1.dev1-py3-none-any.whl: FAILED
2020-10-21T15:07:30.1071735Z   `long_description` has syntax errors in markup and would not be rendered on PyPI.
2020-10-21T15:07:30.1072681Z     line 17: Warning: Inline literal start-string without end-string.
2020-10-21T15:07:30.1073551Z   warning: `long_description_content_type` missing. defaulting to `text/x-rst`.
2020-10-21T15:07:30.1449807Z ##[error]Process completed with exit code 1.
```

The long description is from `README.md` so the `long_description_content_type`
should be `text/markdown`.

setup.py

@@ -17,6 +17,7 @@ def get_dist(pkgname):
     author_email='[email protected]',
     description='Building blocks for recreating darknet networks in pytorch',
     long_description=open('README.md').read(),
+    long_description_content_type='text/markdown',
     # The following settings retreive the version from git.
     # See https://github.com/pypa/setuptools_scm/ for more information
     setup_requires=['setuptools_scm'],