Update README.md

Author: alexisjanvier

README.md

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+<table>
+        <tr>
+            <td><img width="20" src="https://cdnjs.cloudflare.com/ajax/libs/octicons/8.5.0/svg/archive.svg" alt="archived" /></td>
+            <td><strong>Archived Repository</strong><br />
+            This code is no longer maintained. Feel free to fork it, but use it at your own risks.
+        </td>
+        </tr>
+</table>
+
 # JSON REST Client for admin-on-rest
 
 For testing purpose with [admin-on-rest](https://github.com/marmelab/admin-on-rest), use the `jsonRestClient` function that simply takes a JavaScript object, and serves as a regular REST client.