Document AIX requirements to run metronome

[pshipton]

Subject:
See eclipse-openj9/openj9#6499 (comment)

Suggestion:
We used to have the following documentation for setting up for the metronome gc policy, but it was removed.
https://www.ibm.com/docs/en/sdk-java-technology/7.1?topic=collector-configuring-user-accounts

This step must be completed one time only:

When the installation process is completed, you must change the user account to allow access to high-resolution timers. Run the following command as root user:

chuser "capabilities=CAP_NUMA_ATTACH,CAP_PROPAGATE" <username>

where <username> is the non-root AIX user account.

Note: The user must log out and log back in for the change to take effect.

This step must be completed in every shell before starting Java™:

Set the AIXTHREAD_HRT environment variable to true. This environment variable allows a process to use high-resolution timeouts with clock_nanosleep(). You must set this environment variable each time the process is started. On the command line, type:

AIXTHREAD_HRT=true

This setting can be added to a user's .profile so that it is set each time the user logs in. Add the following line to the user .profile file:

export AIXTHREAD_HRT=true