readme (pyproject.toml)
The full description of the project (i.e. the README).
readme = {file = "README.txt", content-type = "text/markdown"}(table)
stringで指定もできる(拡張子からcontent-typeが推論されるらしい)
If the file path ends in a case-insensitive .md suffix, then tools MUST assume the content-type is text/markdown.
If the file path ends in a case-insensitive .rst, then tools MUST assume the content-type is text/x-rst.
Tools MUST assume the file’s encoding is UTF-8.