Documentation TODOs

[adswa]

• [ ] Improve glossary terms on XNAT terminology, maybe enhance with references to BIDS concepts
• [x] explain the provider configuration created by xnat-init, could point to http://handbook.datalad.org/en/latest/beyond_basics/101-146-providers.html (@adswa takes this)
• [x] Authentication section,
  • [x] Authentication in general
  • [x] authentication including the --credential anonymous flag.
  • [x] How to reset wrong credentials
• [ ] Tutorial: Demonstrate the ability for setting up subdataset structures (included in the tests as an example) (@loj takes this)
• [ ] Tutorial: Walk-Through ConnectomeDB
• [ ] Fill in the sections "What can you use the extension for"
• [x] and "What can you not use the extension for"
• [ ] introduction