Updated examples of dataset descriptors

I've added an example of each of the dataset descriptor levels from [W3C HCLS](http://htmlpreview.github.io/?https://github.com/joejimbo/HCLSDatasetDescriptions/blob/master/Overview.html).

- Summary level descriptions
  - Database level
  - Project level
- Version level descriptions
- Distribution level descriptions
  - Component Level
  - Linkset level