docs: document usage of architecture diagrams

assets/DIAGRAM_NOTES.md

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+# Architecture diagram notes
+
+This file documents the purpose of the diagrams stored under `assets/` and how they relate to the Malachite codebase.
+
+## Intended usage
+
+- Architecture diagrams can be embedded in Markdown specs under `specs/` or in external documentation.
+- When updating a diagram, ensure that:
+  - The file name is stable (to avoid breaking links).
+  - The corresponding spec or doc is updated in the same pull request.
+
+## Suggested conventions
+
+- Prefer vector formats (e.g. SVG) when possible.
+- Keep text labels short and use terminology consistent with the rest of the project.
+- When adding a new diagram, include a short caption in the relevant spec that explains what it represents.
+
+## Keeping diagrams in sync
+
+When you make a protocol or API change that affects a diagram:
+
+1. Update the diagram source.
+2. Export the updated asset into this directory.
+3. Update any specs that embed or reference the diagram.
+4. Mention the updated diagrams explicitly in your pull request description.