Add instructions for an authenticated SPARQL endpoint

[ThomasThelen]

Fixes #11

Summary

This PR is fairly lightweight: it's a small addition to the readme that outlines how to add authentication to the SPARQL endpoint.

To Test

1. Deploy slinky
2. Create a user inn Virtuoso. See this guide
3. Follow the README
4. Attempt to login to the SPARQL endpoint
5. You should be prompted with a login field, your credentials should work

[amoeba]

Cool, that looks easy enough to do if we decide to toggle this on. Thanks for researching and adding to the readme.