Add Target as a production user

[Target](http://target.com) is using JanusGraph in production for internal use cases. More information can be found in the second presentation of the JanusGraph meetup №2:
https://training.experoinc.com/janus-meetup-august7/

Read the linked JanusGraph meetup presentation to verify Target's production use and identify the project's existing production-user listing. Locate that documentation entry point, add Target with the supported reference, and confirm the resulting documentation renders correctly.

Written by the indexing model from the issue text.