add simple build instruction

Author: leofang

cuda_py/DESCRIPTION.rst

@@ -49,4 +49,4 @@ packages = ["cuda", "cuda.py"]
 
 [tool.setuptools.dynamic]
 version = { attr = "cuda.py._version.__version__" }
-readme = { file = ["DESCRIPTION.rst"], content-type = "text/x-rst" }
+readme = { file = ["README.md"], content-type = "text/markdown" }