Add link to the example for extension-upload-download-text (#725)

Don't show in the changelog

Author: Bouh

extensions/community/UploadDownloadTextFile.json

@@ -11,7 +11,9 @@
   "version": "1.0.0",
   "description": [
     "Allows users to upload/download text content to a filename and vice versa. ",
-    "(The current gdevelop only supports desktop version, this one is compatible with browsers)"
+    "(The current gdevelop only supports desktop version, this one is compatible with browsers)",
+    "",
+    "[Open this example](https://editor.gdevelop.io/?project=https://resources.gdevelop-app.com/examples/extension-upload-download-text/extension-upload-download-text.json) to learn how to use this extension."
   ],
   "tags": [
     "download",