docs: describe use of pydantic

Author: Midnighter

docs/adr/0009-use-pydantic-for-json-de-serialization.md

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+# 9. Use pydantic for JSON (de-)serialization
+
+Date: 2020-06-09
+
+## Status
+
+Accepted
+
+## Context
+
+In order to interact with a remote workspace, for example, at structurizr.com.
+The remote or local workspace has to be (de-)serialized from or to JSON.
+
+## Decision
+
+In order to perform these operations we choose
+[pydantic](https://pydantic-docs.helpmanual.io/) which has a nice API, active
+community, good data validation, helpful documentation, and good performance.
+
+## Consequences
+
+We separate the models representing Structurizr entities and their business
+logic from how those models are (de-)serialized. That means that for each model
+we have a corresponding IO pydantic model describing the JSON data model.
+