Add documentation for synchronization algorithm

SYNCHRONIZATION.md

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+# ProfiSync™
+
+## Synchronization process
+
+1. Get all local entities(1)
+2. Split into scheduled for deletion(2) with `additionalData=DELETE` and remaining(3)
+3. Execute remote and local deletions of 2
+4. Filter 3 into "new" entities(4) with `id=NEW`
+5. Upload 4 to remote and adjust `remoteID` of entities
+6. Get new list of remaining local entities(5) because `remoteID`s have changed
+7. Get remote entities(6)
+8. Split 6 into merge conflicts(7) that have a matching `remoteID` in 6 and downloaded lists(8)
+9. Store 8 in local database
+10. Pair each entity from 7 with the entity with matching `remoteID` from 6(9)
+11. Merge the pairs from 9 to a single entity(10) and update the timestamp in `additionalData`
+    * If a property only exists locally, use the local property and vice versa
+    * If a property exists both locally and remotely then use the values from the entity with higher timestamp
+12.  Compare server-relevant properties of each item of 10 with the remotely stored version and update if different
+13. Compare client-relevant properties of each item of 10 with the locally stored version and update if different
+    * The comparison could be omitted because SQLite performance is not a concern
+
+## Common functionality for entities
+
+* `remoteID` and `additionalData`
+* server-relevant comparison
+* client-relevant comparison
+* database crud
+* remote crud
+* merging