Help files for XLS import and export

Added some help for XLS (excel file) schema and data import and data
export. Added them to the doc.

Change-Id: I77abc4a17a52ae971c06a847db12f54722b4bdc3
Reviewed-on: http://gerrit.esdi-community.eu/3140
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>