Adding a documentation about dynamic resource allocation.

Signed-off-by: Tsuyoshi Ozawa <ozawa.tsuyoshi@gmail.com>
Fixing a documentation of spark.dynamicAllocation.enabled.

Signed-off-by: Tsuyoshi Ozawa <ozawa.tsuyoshi@gmail.com>
Merge branch 'SPARK-4839' of github.com:oza/spark into document-dynamic-allocation
Document the configs as well