Address warning: `long_description_content_type` missing. defaulting to `text/x-rst`. error

[pombredanne]

twine check returns this on package wheels and sdist

[pombredanne]

For instance:

$ twine check release/pypi/*
Checking release/pypi/scancode_toolkit-30.1.0-py3-none-any.whl: PASSED, with warnings
  warning: `long_description_content_type` missing. defaulting to `text/x-rst`.
Checking release/pypi/scancode_toolkit_mini-30.1.0-py3-none-any.whl: PASSED, with warnings
  warning: `long_description_content_type` missing. defaulting to `text/x-rst`.
Checking release/pypi/scancode-toolkit-30.1.0.tar.gz: PASSED, with warnings
  warning: `long_description_content_type` missing. defaulting to `text/x-rst`.
Checking release/pypi/scancode-toolkit-mini-30.1.0.tar.gz: PASSED, with warnings
  warning: `long_description_content_type` missing. defaulting to `text/x-rst`.