sync doc notes

SYNC.md

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+# sync
+
+Notes on the synchronization process by Todoist.
+
+## How do edited, completed, deleted, rescheduled and uncompleted items respond?
+
+Edited: Full object
+Completed: `is_completed`, full object
+Deleted: `is_deleted`, full object
+
+TODO: Rescheduled, uncompleted
+
+## How should synchronized items be identified/updated?
+
+The `id` detail.
+
+TODO: It is not well specified in the docs what the meaning of `v2_id` is.
+
+## What data structure should be used to store synchronized items?
+
+A dictionary with the `id` as the key.
+
+## How and when should synchronization be triggered?
+
+With the cron schedule, first thing.
+
+## Is there a limit to the delay between synchronization?
+
+Likely: if there is a limit, it should be handled automatically by the `full_sync` attribute.