Rewrite benchmark doc

## Due Date
*To be completed by:* 2018-04-30

## Description
Our doc on benchmarks lacks a link to the actual repository, and does not include any helpful directions on running the benchmarks from the repository. This will need a total rewrite.

https://deeplearning4j.org/benchmark

## Assignees
@eraly because you aren't familiar with the actual repository, can you help write this? We need to make sure that this works for users who are totally new to benchmarks (and benchmarking in general).