update README

Author: aurelienp-pdp

README.md

@@ -1 +1,6 @@
-# UniversalBusinessLanguage
+# UniversalBusinessLanguage
+
+XSD are from this repository : https://github.com/ConnectingEurope/eInvoicing-EN16931/tree/master/ubl/schema
+
+The XML Schema (XSD) of OASIS UBL 2.1 can be downloaded from OASIS.
+The ZIP file is located at http://docs.oasis-open.org/ubl/os-UBL-2.1/UBL-2.1.zip