Documentation - Include Guidelines on Using Sail to load RDF data files

How are RDF data files loaded into JanusGraph? Is there any way to stand-up a SPARQL endpoint on a JanusGraph stack? Please see related discussion referencing Sail at
- https://groups.google.com/forum/#!topic/gremlin-users/nIE6uaSck8g

Start by reading the related Gremlin Users discussion linked in the issue, then verify JanusGraph's current guidance for loading RDF data files with Sail and standing up a SPARQL endpoint. Done means the documentation answers both questions with supported steps and clearly identifies any limitations.

Written by the indexing model from the issue text.