I am running javadoc with ant task:
You're looking for the -overview option of javadoc. See http://docs.oracle.com/javase/6/docs/technotes/tools/windows/javadoc.html#overviewcomment for details.
In Ant, you need to use the overview attribute like so:
overview
<javadoc destdir="javadoc" overview="src/overview.html">...</javadoc>