encourage RST checking with readme_renderer

[svenevs]

python setup.py check --restructuredtext did not report any errors, but readme_renderer explained effectively and concisely that .. raw:: directives are not allowed.

1. I didn't know what to do for the admonition. I think it's in the right section, but there was already a note and a warning, and the pypa_theme doesn't seem to stylize tip or danger (sphinx only?).
   • I could also make it a definition list under **Tip**?
2. Happy to change any and all verbiage. I just feel like linking to this tool directly will save a lot of people time, and reduce burden on PyPI from people who are releasing versions just to try and get the long description to show up.
   • I'll confess this was me, but internet trolling seems to reveal it's not uncommon. I can't use testpypi for some reason, so I didn't really have any options.
   • Then I happened upon this magical tool, and it told me exactly why I couldn't use it. TBH I'm surprised GitHub even let my .. raw:: html through...
3. I specifically said "not Sphinx" because I think that's what most people assume it would be.
   • Should I link to Sphinx?
   • Should I state that it is the (golden standard) docutils parser? (is that even correct)?

Let me know if you think this is worth including, but the verbiage should change!

[theacodes]

Thanks, @svenevs!