docs: use README.md on Pypi

Author: raphael0202

README.rst

@@ -4,7 +4,7 @@ version = attr: openfoodfacts.__version__
 author = The Open Food Facts team
 author_email = [email protected]
 description = Official Python SDK of Open Food Facts
-long_description = file: README.rst
+long_description = file: README.md
 license = Apache 2.0
 classifiers =
     Development Status :: 5 - Production/Stable