Added Mindtagger example and documentation

Please take a look at the new documentation on using Mindtagger, and comment on anything you want to see improved.  Here's a link to easily preview it: <https://github.com/HazyResearch/deepdive/blob/netj-mindtagger-example/doc/doc/basics/labeling.md#readme>

I'm thinking of adding a few more links to/from the existing documentation, e.g., walkthrough, but I'm fine with doing that in another PR and getting this in.  Currently the new page is only linked from the index.