Fix #49 by adding note about recommended format for ISCED code

endpoints/get-response.xsd

@@ -480,6 +480,10 @@
                         ISCED-F codes define fields of education and training at the secondary,
                         post-secondary and tertiary levels of education. Details:
                         http://www.uis.unesco.org/Education/Documents/isced-fields-of-education-training-2013.pdf
+
+                        Note: We strongly recommend that the ISCED code should be four-digit,
+                        because this way it will be compliant with the requirements of Mobility Tool+ application,
+                        which only accepts four-digit codes for mobilities (see issue: #49).
                     </xs:documentation>
                 </xs:annotation>
             </xs:element>