DOC: README: symlink, format as rst like the docs

[westurner]

• https://github.com/tux4kids/tuxmath/blob/master/README
  • rm README; ln -s doc/README README
  • mv README README.rst
• https://github.com/tux4kids/tuxmath/blob/master/doc/README
  • https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax

[westurner]

• It would be DRY to include README.rst in doc/README.rst like cookiecutter-pypackage, for example: https://github.com/audreyfeldroy/cookiecutter-pypackage/blob/master/%7B%7Bcookiecutter.project_slug%7D%7D/docs/readme.rst :
  .. include: ../../README.rst