Add documentation about auto-adding source bucket

docs/Catalog/Preferences.md

@@ -99,8 +99,18 @@ ui:
   defaultSourceBucket: s3://bucket-b
 ```
 
+Note that the `ui.sourceBuckets` is empty by default.
+As a result, when users create or revise a package in a new bucket
+they can't add files from any bucket, including that one.
+Instead, you need to manually add a configuration file,
+or click "auto-add current bucket"
+(which will create or edit the configuration file to add
+the current bucket to the list of `ui.sourceBuckets`)
+
+![Users can auto-add the current bucket to ui.sourceBuckets](../imgs/auto-add-source-bucket.png)
+
 #### `ui.package_description` example
-  
+
 ```yaml
 ui:
   package_description: