[docs] Add document about add column at last.

Author: loserwang1024

website/docs/engine-flink/ddl.md

@@ -247,7 +247,19 @@ The following example illustrates reset the `table.datalake.enabled` option to i
 ```sql title="Flink SQL"
 ALTER TABLE my_table RESET ('table.datalake.enabled');
 ```
+### Add Column At Last
+Currently, Fluss only supports adding Nullable columns at the end of the table.
+Below are examples demonstrating how to add columns:
+```sql title="Flink SQL"
+-- Add a single column at the end of the table
+ALTER TABLE my_table ADD user_email STRING COMMENT 'User email address';
 
+-- Add multiple columns at the end of the table
+ALTER TABLE MyTable ADD (
+    user_email STRING COMMENT 'User email address',
+    order_quantity INT
+);
+```
 
 ## Add Partition
 

website/docs/maintenance/operations/upgrade-notes-0.9.md

@@ -7,6 +7,16 @@ sidebar_position: 4
 
 These upgrade notes discuss important aspects, such as configuration, behavior, or dependencies, that changed between Fluss 0.8 and Fluss 0.9. Please read these notes carefully if you are planning to upgrade your Fluss version to 0.9.
 
+## Add Column At Last.
+If you're only upgrading to Fluss v0.9 without modifying table schemas, you can safely skip this section.
+
+However, If you plan to add columns to existing tables, follow these mandatory steps:
+1. Upgrade all Fluss servers to v0.9 
+Ensure complete server infrastructure is running v0.9 before attempting any schema modifications.
+2. Restart and upgrade Fluss client to v0.9
+Old clients are incompatible with mixed schema versions and cannot properly read data with different schemas. This includes flink job, tier service and java client.
+
+**Skipping these steps when adding columns may result in schema inconsistency issues or data inaccessibility.**
 
 ## Deprecation / End of Support