Updated [math] specific release howto.

 - added references to the general Apache Commons release howtos,
 - changed the staging area of the distribution files to the Apache
   servers (and not the Nexus repository),
 - added the URL of the site for direct subversion control

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1465382 13f79535-47bb-0310-9956-ffa450edef68