Documentation of DataSet transformations (Work In Progress)

Current State:

- First Version of DataSet transformation documentation
- Included: Map, FlatMap, Filter, Project, Reduce (All, Grouped), GroupReduce (All, Grouped), Join
- Missing: Aggregate, Cross, CoGroup, Union, good code examples for GroupReduce (suggestions welcome)
- Not spellchecked yet!

I will continue to finish the documentation.
Comments are welcome!