Update node utilization documentation

[jtpalmer]

Description

Updates the node utilization documentation with instructions on how to add the statistic.

Motivation and Context

The node utilization statistic is not enabled by default and this result in it's removal during the group by refactoring.

https://app.asana.com/0/342819846538629/1185671232325737/f

Tests performed

Tested instructions manually.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[plessbd]

Thank you for doing this, should we make this more "generic" at some point as a "How to add additional metrics"

Just an idea.

[plessbd]

We need to add something about doing a touch on a file inside /etc/xdmod/datawarehouse.d/ or restarting the web server.
Otherwise APC wont pick up the change

[jtpalmer]

@plessbd are you sure about that? I tested the instructions and didn't need to do that.

[jtpalmer]

@plessbd I updated the instructions.