Fix long description associated pypi package

[jcfr]

Instead of:

https://github.com/InsightSoftwareConsortium/ITKColorNormalization/blob/b65eb0f8f16ee77dfb2c5b8720acf85a6d6a5b96/setup.py#L23

Consider doing something like this:

with open('README.md', 'r') as fp:
    readme = fp.read()

and add the following arguments to the setup function:

    long_description=readme,
    long_description_content_type='text/markdown',

For example, see https://github.com/CastXML/CastXML-python-distributions/blob/master/setup.py

[thewtex]

:+1: good idea @jcfr .

After implemented here, this should be added to ITKModuleTemplate.