support more use-cases, and document them nicely

[martinjaggi]

usability will be a key aspect to make this project work

pystruct has done a good job documenting some nice use-cases for example.
though as we discussed, we should focus on a smaller number of highly relevant ones. we can try to collect a useful list here.

first maybe let's showcase some nice things we can already do with the current oracles we have, e.g.

• NLP with chain oracle (chunking / POS tagging / entity linking?)
• vision with a chain oracle (we already have OCR, but that will not convince the vision community yet)
• bio informatics?