## What changes were proposed in this pull request?

We should mention that users can build Spark using multiple threads to decrease build times; either here or in "Building Spark"

## How was this patch tested?

Built on machines with between one core to 192 cores using mvn -T 1C and observed faster build times with no loss in stability

In response to the question here https://issues.apache.org/jira/browse/SPARK-15821 I think we should suggest this option as we know it works for Spark and can result in faster builds

Author: Adam Roberts <aroberts@uk.ibm.com>

Closes #13562 from a-roberts/patch-3.