Add note about requirements for Markdown descriptions

[di]

A lot of users are getting bit when first trying to publish when their first Markdown description because various tools they're using are out of date:

• Long Description Not Rendered as Markdown pypi/warehouse#3849
• long_description_content_type=text/markdown doesn't always work pypi/warehouse#3664
• and elswhere

When we're talking about long_description_content_type or Description-Content-Type in this guide, we should probably add a note or caveat about requiring the following versions for it to work:

• setuptools>=38.6.0
• wheel>=0.31.0
• twine>=1.11.0

[pgadige]

I'm working on this.
I think I'll add a note saying that

When you are writing your project's description using Markdown on PyPI, ensure that the versions of the following are used:

• setuptools>=38.6.0
• wheel>=0.31.0
• twine>=1.11.0

before the box with the example code.
Could you please review the content that should go into the note box?
@di and @theacodes, may I know your thoughts about this one?
Thank you!

[di]

Given the confusion with pypi/warehouse#4099, we should probably also indicate that the upload must be performed with twine, and not with python setup.py upload.

We could also provide a helpful command to run to ensure all versions are up to date, like pip install -U setuptools wheel twine.

[jarekwg]

^ Yes please!
Alongside version requirements, the actual commands should be clearly listed:

python setup.py bdist_wheel
twine upload dist/<new_package_name>

Took me a silly amount of stack overflow digging to figure this out..

[pgadige]

I drafted a concise note including the above details:

If you're using GitHub flavored Markdown to write a project's description on PyPI, ensure you update the following tools:

 `python3 -m pip install --user --upgrade setuptools wheel twine`

The preferred versions of the tools:

• setuptools>=38.6.0
• wheel>=0.31.0
• twine>=1.11.0

It's recommended you use twine to upload the project's distribution packages:

`twine upload dist/*`

Could you please review the content and the structure of the draft, @di and @theacodes ?
I appreciate your feedback and suggestions.

[theacodes]

Looks good enough to start with, send a PR?

[pgadige]

@theacodes and @di, I added a note at what I thought would be an appropriate place in the existing doc. I'll appreciate your feedback, suggestions and corrections to be made. Thank you!