Provide details on "Users" listing requirements

On the main [JanusGraph](https://janusgraph.org/#users) web site, there is a section for **Users** (The following users have deployed JanusGraph in production). It would be great to provide a page that describes the requirements to be listed. I'd imagine that the process would be pretty simple, along the lines of:

- Send an email to janusgraph-dev with a short description of the product's JanusGraph use case, including the product name and version that uses it
- 72 hour lazy consensus by committers
- ~~Provide clear external documentation of your product's usage of JanusGraph, including proper attribution and linkage to the JanusGraph project~~
- ~~Approval vote is required~~

For reference, here are the [Apache TinkerPop](http://tinkerpop.apache.org/policy.html) index listing requirements:

- The project must be either a TinkerPop-enabled graph system, a Gremlin language variant/compiler, a Gremlin language driver, or a TinkerPop-enabled middleware tool.
- The project must have a public URL that can be referenced by Apache TinkerPop.
- The project must have at least one release.
- The project must be actively developed/maintained to a current or previous "y" version of Apache TinkerPop (3.y.z).
- The project must have some documentation and that documentation must make explicit its usage of Apache TinkerPop and its version compatibility requirements.

Start at the JanusGraph website's Users section and review the issue's proposed listing process alongside the referenced Apache TinkerPop requirements. Add a page explaining how products can be listed, including the requested submission and consensus steps, then verify that the Users section links to it.

Written by the indexing model from the issue text.