Add missing information for the customized files importing doc

[shenli]

File: /release-6.1/tidb-lightning/tidb-lightning-data-source.md

Importing customized csv files needs more configuration, such as the pattern of table/schema DDL file name, such as:
`
[[mydumper.files]]
pattern = '^(test1).(tbl1)-schema.sql'
schema = "$1"
table = "$2"
type = "table-schema"

[[mydumper.files]]
pattern = '^(test1)-schema-create.sql'
schema = "$1"
type = "schema-schema"
`
Without the patterns for table/schema DDL file, users will meet a "schema not found" error if the table/schema are not created in advance.

[ran-huang]

According to @niubell, this is by design. If there is no schema file and the table has not been created beforehand, an error message will be displayed to inform the user.
cc @Frank945946