Provide documentation and/or code samples for bulk loading

There have been a number of questions about [bulk loading on the mailing list][1], as well as in GitHub issues, such as #268 and #602.

This is also a critical step in the getting started process for using a new database such as JanusGraph, and in many cases, users' first experience with using JanusGraph for a non-trivial exercise, so it's important to make this a smooth and simple process.

JanusGraph users would benefit from having:

* simple and clear documentation for how to do bulk import of data, e.g., CSV/TSV, etc. into JanusGraph
* whether there are any backend-specific optimizations or tuning parameters for bulk loading
* code samples that users can either run as-is with just a few parameters, or build upon for custom bulk loading

[1]: https://groups.google.com/forum/#!searchin/janusgraph-users/bulk$20load%7Csort:relevance

Start by reviewing the linked mailing-list discussion and related issues #268 and #602 to identify the unresolved bulk-loading questions. Done should provide clear CSV/TSV import guidance, explain backend-specific tuning, and include runnable code samples with only a few required parameters.

Written by the indexing model from the issue text.