[SPARK-15821][DOCS] Include parallel build info

## 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.

README.md

@@ -25,6 +25,8 @@ To build Spark and its example programs, run:
     build/mvn -DskipTests clean package
 
 (You do not need to do this if you downloaded a pre-built package.)
+
+You can build Spark using more than one thread by using the -T option with Maven, see ["Parallel builds in Maven 3"](https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3).
 More detailed documentation is available from the project site, at
 ["Building Spark"](http://spark.apache.org/docs/latest/building-spark.html).
 For developing Spark using an IDE, see [Eclipse](https://cwiki.apache.org/confluence/display/SPARK/Useful+Developer+Tools#UsefulDeveloperTools-Eclipse)

dev/make-distribution.sh

@@ -53,7 +53,7 @@ while (( "$#" )); do
     --hadoop)
       echo "Error: '--hadoop' is no longer supported:"
       echo "Error: use Maven profiles and options -Dhadoop.version and -Dyarn.version instead."
-      echo "Error: Related profiles include hadoop-2.2, hadoop-2.3 and hadoop-2.4."
+      echo "Error: Related profiles include hadoop-2.2, hadoop-2.3, hadoop-2.4, hadoop-2.6 and hadoop-2.7."
       exit_with_usage
       ;;
     --with-yarn)
@@ -150,7 +150,7 @@ export MAVEN_OPTS="${MAVEN_OPTS:--Xmx2g -XX:MaxPermSize=512M -XX:ReservedCodeCac
 # Store the command as an array because $MVN variable might have spaces in it.
 # Normal quoting tricks don't work.
 # See: http://mywiki.wooledge.org/BashFAQ/050
-BUILD_COMMAND=("$MVN" clean package -DskipTests $@)
+BUILD_COMMAND=("$MVN" -T 1C clean package -DskipTests $@)
 
 # Actually build the jar
 echo -e "\nBuilding with..."