added comment

ch.psi.fda.xscan/src/main/java/ch/psi/fda/core/loops/cr/MainHeader.java

@@ -1,5 +1,8 @@
 package ch.psi.fda.core.loops.cr;
 
+/**
+ * Header of the ZMQ message
+ */
 public class MainHeader {
 	private String htype;
 	private int elements;