Add readme and class diagram

hexagonal/README.md

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+---
+layout: pattern
+title: Hexagonal Architecture
+folder: hexagonal
+permalink: /patterns/hexagonal/
+categories: Architectural
+tags:
+ - Java
+ - Difficulty-Expert
+---
+
+## Intent
+Allow an application to equally be driven by users, programs, automated test or batch scripts, and to be developed and tested in isolation from its eventual run-time devices and databases.
+
+![Hexagonal Architecture class diagram](./etc/hexagonal.png)
+
+## Applicability
+Use Hexagonal Architecture pattern when
+
+* it is important that the application is fully testable
+* you use Domain Driven Design methodology and/or Microservices architectural style
+
+## Real world examples
+
+* [Apache Isis](https://isis.apache.org/)
+
+## Credits
+
+* [Alistair Cockburn - Hexagonal Architecture](http://alistair.cockburn.us/Hexagonal+architecture)