Add description that will be displayed on PyPI

Author: rayluo

setup.cfg

@@ -9,3 +9,4 @@ project_urls =    Changelog = https://github.com/AzureAD/microsoft-authenticatio
 classifiers =
     License :: OSI Approved :: MIT License
     Development Status :: 4 - Beta
+description = Microsoft Authentication Library extensions (MSAL EX) provides a persistence API that can save your data on disk, encrypted on Windows, macOS and Linux. Concurrent data access will be coordinated by a file lock mechanism.