Docs: Add description for config fields according to SINGLE-OBJECT

[Siemienik]

sibling of #62

Actual

Described only shared cfg fields.

Expected

Following link: https://github.com/Siemienik/xlsx-import#fields-or-columns

Should be added section Single object depended fields with information: "Aliases: single, object, singleton".
The section should contains fields description (from IObjectSourceConfig and ISourceConfig): https://github.com/Siemienik/xlsx-import/blob/0db404b6be0117f3f81ede01879b1bbed3a4a3d8/src/config/IObjectSourceConfig.ts#L4-L6

[Siemienik]

close as done #146