README: adds overlap code as extra vlkb-specific feature

README.md

@@ -3,6 +3,7 @@
 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:
 
+- provides overlap code
 - 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