Add a guide that start an incremental task with different table strcuture in upstream and downstream

[lance6716]

Change Request

This repository is ONLY used to solve issues related to TiDB Data Migration (DM) documentation.
For other issues (related to TiDB, PD, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

Sometimes it's needed to create a downstream table that different with upstream table, for exmaple, there's conflicting PK in upstream sharding tables so downstream one should add another column to build a compsite index.

2. Describe your suggestion or addition.

as title

3. Provide some reference materials (documents, websites, etc) if you could.