Add header to result file to explain what the numbers mean
Add header to result file to explain what the numbers mean

It was very confusing before. For each test all we see is something
like 10.1, 0.25, 9.75, 10.8, 10.35. Every time I have to dig into
the code to figure out what these mean. This commit adds a header to
the result file to explain this.

arijitt/spark-perf,XiaoqingWang/spark-perf,Altiscale/spark-perf,jkbradley/spark-perf,nchammas/spark-perf,XiaoqingWang/spark-perf,nchammas/spark-perf,mengxr/spark-perf,arijitt/spark-perf,feynmanliang/spark-perf,Altiscale/spark-perf,zsxwing/spark-perf,jkbradley/spark-perf,mengxr/spark-perf,databricks/spark-perf,zsxwing/spark-perf,feynmanliang/spark-perf,databricks/spark-perf