Add a prompt explaining the situation as well as documentation

docs/gdrive-userscript-serveradmins.md

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+# Google Drive Userscript Setup
+
+In response to increasing difficulty and complexity of maintaining Google Drive
+support, the native player is being phased out in favor of requiring a
+userscript to allow each client to fetch the video stream links for themselves.
+Users will be prompted with a link to `/google_drive_userscript`, which explains
+the situation and instructs how to install the userscript.
+
+As a server admin, you must generate the userscript from the template by using
+the following command:
+
+```sh
+npm run generate-userscript <site name> <url> [<url>...]
+```
+
+The first argument is the site name as it will appear in the userscript title.
+The remaining arguments are the URL patterns on which the script will run.  For
+example, for cytu.be I use:
+
+```sh
+npm run generate-userscript CyTube http://cytu.be/r/* https://cytu.be/r/*
+```
+
+This will generate `www/js/cytube-google-drive.user.js`.