Update README.md

Envelope/Use-Cases/README.md

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 Most of the following use cases are offered progressively: additional use cases build on earlier ones, expanding the fundamental ideas with new functionality in each example. (That functionality is listed as part of each use case's name.)
 
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+For a broader overview of these use cases and some discussions of what they have in common, see our [blog posting on Gordian Envelopes](https://www.blockchaincommons.com/introduction/Gordian-Envelope-Use-Cases/).
 
 ## [Educational & Credential Industry Use Cases](https://github.com/BlockchainCommons/Gordian/blob/master/Envelope/Use-Cases/Educational.md)