Add README for the project

Author: Yevhen Zavhorodnii

README.md

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+# Structurizr Threagile Exporter
+
+The [ThreagileExporter](/lib/src/main/java/com/ticketmaster/structurizr/threagile/ThreagileWorkspaceExporter.java) class provides a way
+to export Structurizr views to diagram definitions that are compatible with [Threagile](https://github.com/Threagile/threagile).
+
+This library is developed to be included in the [Structurizr CLI](https://github.com/structurizr/cli),
+and is available on Maven Central, for inclusion in your own Java applications:
+
+- groupId: `com.ticketmaster.structurizr`
+- artifactId: `threagile`