Missing gtid tutorial in add-tables-to-existing-task

[GMHDBJD]

Error Report

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. What is the URL/path of the document related to this issue?
   Missing gtid tutorial in add-tables-to-existing-task

2. How would you like to improve it?
   add tutorial later