Update README.md

Author: jasonjoh

README.md

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-# Microsoft Graph UWP Snippets Sample (SDK)
+# [ARCHIVED] Microsoft Graph UWP Snippets Sample (SDK)
+
+## IMPORTANT
+
+**This project is being archived and replaced with the [Build UWP apps with Microsoft Graph](https://github.com/microsoftgraph/msgraph-training-uwp). As part of the archival process, we're closing all open issues and pull requests.**
+
+**You can continue to use this sample "as-is", but it won't be maintained moving forward. We apologize for any inconvenience.**
 
 **Table of contents**