update readme

pyproject.toml

@@ -59,7 +59,7 @@ license-files = ["LICENSE"]
 
 [tool.setuptools.dynamic]
 dependencies = {file = "requirements/core.txt"}
-readme = {file = "README.rst", content-type = "text/x-rst"}
+readme = {file = "README.md", content-type = "text/markdown"}
 version = {attr = "iris_grib.__version__"}
 optional-dependencies.all = {file = "requirements/all.txt"}
 optional-dependencies.test = {file = "requirements/test.txt"}