Create readme.md

Author: peterli-r3

BusinessNetwork-Extension/readme.md

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+## Business Network Extension Cordapp Sample
+
+This folder features sample projects that utilize bn-extension.
+
+### [Insurance Business network](./flow-database-access):
+This CorDapp provides a simple example of how a mock business network is set up with bn-extension. It consists of one sub group and two custom network identities, and multiple roles associated with each identity.