please add use cases or link to them

[debpaul]

@godfoder please upload Barb's NCD use cases to this folder: https://github.com/tdwg/ncd/tree/master/crosswalks-related-docs

Then we can link her work here https://github.com/tdwg/ncd/wiki/Crosswalks-and-Use-Cases#use-cases-for-ncd

[stanblum]

• Index Herbariorum use-cases from Barbara Thiers uploaded as a markdown file;
• renamed to use-cases.md;
• headings adjusted so that other use-cases can be added from other perspectives;
• may want to reformat / restructure these

[debpaul]

many thanks @stanblum