DOC: docstring added for leap seconds initialization procedure

src/main/java/caosdb/datetime/UTCDateTime.java

@@ -162,6 +162,11 @@ public class UTCDateTime implements Interval {
     addLeapSecond(gc.getTimeInMillis() / 1000);
   }
 
+    /**
+     * Initialize the container with known leap seconds. This container has to be updated according to future changes of leap seconds.
+     *
+     * Currently all leap seconds between 1972 and 2019 are registered here, with the most recent leap second being added for December 2016.
+     */
   private static void initLeapSeconds() {
 
     // june 1972