Document warnings on how to set (long_)description

I have just spent >= 1 hour to overcome this bug: https://github.com/pypa/twine/issues/454

• In the description argument passed to setup(...), you must not pass newlines!
  
  (Especially I passed __doc__, which starts of course with a newline most of the time.)
• In the long_description argument passed to setup(...), you must use LF newlines (\n) exclusively. (No \n! No \r!)
  
  I suggest doing

  # Open readme with original (i.e. LF) newlines
  # to prevent the all too common "`long_description_content_type` missing"
  # bug (https://github.com/pypa/twine/issues/454)
  with open('README.md', 'r', newline='', encoding='utf-8') as readme_file:
  	long_description = readme_file.read()
  	long_description_content_type = 'text/markdown'
  
  setup(
  	# ...
  	long_description = long_description,
  	long_description_content_type = long_description_content_type,
  )
  

This should be documented in https://packaging.python.org/tutorials/packaging-projects/.

Start with the packaging-projects tutorial named in the issue and review the linked Twine issue for the reported behavior. Done means the tutorial documents the description and long_description newline requirements and includes guidance for setting long_description_content_type.