Add extra information to enable A-MQ web console login in Installation Guide

The Installation guide of JBoss A-MQ 6.0 EA at URL
https://access.redhat.com/knowledge/docs/en-US/JBoss_A-MQ/6.0.EA/html-single/Installation_Guide/index.html
Chapter 4 step 6.

6. In a Web browser, navigate to http://localhost:8181/activemqweb to open the JBoss A-MQ Web console.

This doesn't work out of the box. I would recommend to add the following before this step.

Open InstallDir/etc/system.properties in your favorite text editor.

Locate the lines
#activemq.jmx.user=admin
#activemq.jmx.password=admin

This line specifies a user admin with the password admin.

Uncomment these lines by removing the leading #.

Change the admin to the desired password.

Save the changes.

Furthermore the system.properties contains the following lines
#

1. Authentication - must match an entry in user.properties with the admin privilege
   #
   should be
   #
2. Authentication - must match an entry in users.properties with the admin privilege
   #