add: overview document with abstract and status sections
add: primer document describing nidm
add: document describing the nidm dataset descriptor
add: document describing the nidm experiment hierarchy
add: start to an abstract
add: start title
fix: formatting
add: revised section on family of specs
add: intro text and additional sections
add: text up to figure 1
add: nidm organization figure
fix: cropped image and added another bullet point
add: finished first draft of into section
add: first draft of the document roadmap
add: first draft of the namespaces section
add: first draft of the additional information is tbd
add: acknowledgements and rm unneeded class declarations
add: nidm-overview as a template to start from
add: prov key concepts diagram
add: abstract draft