Document the google drive sync process

docs/google-drive-sync.md

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+# Google Drive Sync
+
+The bot relies on an external sync between the local file system and Google
+Drive in order to get newer cards to the bot. This is done using
+[Rclone](https://rclone.org/).
+
+The process for this is done by once the `/gdrivesync` command is executed by
+an admin user of the bot, which calls the system shell to run rclone to the
+card folder.
+
+- The admins who can run the command is specifed in `$BOT_ADMINS`, which are
+discord user ids separated by commas.
+- The card folder is located at `$DATA_DIR/cards`.
+- The source requires rclone's remote to be setup as `card-drop-gdrive`.
+
+The exact command it runs is: `rclone sync card-drop-gdrive: $DATA_DIR/cards`.
+
+Once it syncs the database will reread all the cards for updates and then load
+them into the bot to be given.
+
+## Safe Mode
+Safe mode is a function of the bot which disables the `/drop` command function
+and any other functions which rely on the card metadata. Safe mode is activated
+upon failure to sync properly. It is disabled once errors are resolved.
+
+The reason for safe mode is to ensure that the bot stays online for admins to
+be able to resync the bot in case there's an error without it crashing.
+
+## Google Drive
+Please see the Rclone documentation on how to setup a remote using Google
+Drive. You will need to make an app password for this.
+
+- scope: `drive.readonly`
+- root\_folder\_id: The folder id where the cards are located, this can be found
+  by looking at the url when viewing the folder in the browser in google drive.