More clearly defined expected nodes and links data

[peteruithoven]

This one kind of builds on #22. Node groups clearly states "should be given in the following format" which then gives a clear example.

I noticed that the expected format for links, nodes data on the other hand is kind of undocumented. Especially since there are properties like the following which are otherwise only documented elsewhere.

• links → type Used in default linkTitle
• nodes → direction Used in default nodeBackwards

[peteruithoven]

I'd be happy to do a PR, but I'd appreciate some additional info, corrections and/or confirmations first.