Add user guide for groupby apply#29

BryanCutler merged 1 commit intoBryanCutler:arrow-user-docs-SPARK-2221BryanCutler/spark:arrow-user-docs-SPARK-2221from

icexelloss:arrow-user-docs-SPARK-2221icexelloss/spark:arrow-user-docs-SPARK-2221Copy head branch name to clipboard