README: adds a list of extra VLKB-features over standard SIAv2

README.md

 ## vlkb-search
 
-is a web-application to discover astronomical data stored in FITS-files and describe in a VO ObsCore table.
-It consists of
+is a web-application to discover astronomical data stored in FITS-files and described in a VO ObsCore table.
+It implements the SIA version 2, IVOA recommendation (https://ivoa.net/documents/SIA/20151223/) with optional VLKB-specific features:
 
-- **vlkb-search** web-application and associated **vlkb-obscore** command line tool to create the VO ObsCore
+- the filtering paramters (POS, BAND) can be given in GALACTIC and VELO-LSRK-km/s coordinates systems
+- handles addtional metadata about the surveys (name, species, transformation)
+- VOTable response includes composed URLs for file download, file-cutout and files-demosaic services
+
+The service consists of **vlkb-search** web-application and associated **vlkb-obscore** command line tool to create the VO ObsCore
 table from FITS-files.
 
 The vlkb-obscore cli is available from https://ict.inaf.it/gitlab/butora/vlkb-datasets .