Provide a description for `flit-core` on PyPI

[scdub]

I noticed flit-core as a new dependency of typing_extensions 4.4.0. Wanting to see what this package is, I headed over to the PyPI page but got the somewhat ominous "The author of this package has not provided a project description" on that page. Of course, reading it and seeing the subtitle along with the links it isn't hard to figure out, but would be nice to have a description of flit-core directly on PyPI for upstream packages which use the core package for their build backend.

[takluyver]

Makes sense. I think after various other changes, if we list the README file in the appropriate place in its pyproject.toml, that should just work. Do you want to make a PR?