[DOC] Document Proper Use Of ENTITY Tag For activemq.xml Cfg File

The proper use of the ENTITY tag needs to be documented so that the activemq.xml file can include external XML files via the use of that tag.

Here's how one can get it to consistently work:

1. Putting the destinations.xml file in the .../etc directory and using a relative path as follows (note use of file: URI):

<!DOCTYPE beans [
<!ENTITY destinations SYSTEM "file:etc/destinations.xml">
]>

2. Putting the destinations.xml file in a directory outside the installation (e.g., /tmp) and using an absolute path.

<!DOCTYPE beans [
<!ENTITY destinations SYSTEM "file:///tmp/destinations.xml">
]>