Add diagram output folder

This is where the diagrams will be generated.
A readme is included for explanatory purposes.

docs/diagrams/readme.md

@@ -0,0 +1,18 @@
+Diagrams in this directory are auto-generated.
+Any PR that makes changes in the `docs/workspace.dsl` file in this repo will
+trigger a GitHub action that updates the diagrams.
+This will add a new commit to the PR.
+
+# Contributing
+
+If you want to edit the `workspace.dsl` and see your changes reflected during
+testing you have two options:
+
+1. The [online editor](https://structurizr.com/dsl) from Structurizr
+2. The [CLI](https://docs.structurizr.com/cli) from Structurizr
+
+The diagrams are built according to the [C4 model](https://c4model.com/).
+This allows architectural diagrams to be constructed at different levels of abstraction from a single `workspace.dsl`.
+
+If contributing to the diagrams you should be familiar with the C4 Model.
+Any change you make could potentially impact multiple diagrams.