Document ActiveMQ memory requirements

ActiveMQ may need more heap allocated, otherwise it may run out of memory under heavy load:

https://groups.google.com/a/opencast.org/forum/#!topic/users/aLj-ZK6doOM

Update the max heap setting from 1024m to 2048m in /opt/activemq/bin/linux-x86-64/wrapper.conf (ActiveMQ may not be installed in the same place):