The metadata was using README.rst, as a relic of this project's origins,
but we actually use a Markdown file so I'm changing it to README.md.

I looked into the source of pytest-checkdocs and found that it only acts
on Restructured Text files, not on Markdown files. Since we use Markdown
for our README (which is also the long description), it's not doing us
any good, and therefore I'm removing it.

I examined a few different Markdown linters that supply pre-commit hooks
to find one that would work for our project. The one by Igor Shubovych
seems to be popular and actively developed, supports configuration, and
offers auto-fixing of some issues, so I think it's a good choice for
this project.

These changes allow the markdownlint-fix pre-commit hook to pass. These
particular errors are apparently not automatically fixable.

This is the typical disclaimer about how the project isn't done yet.