docs: add database schema

Author: darrachequesne

README.md

@@ -17,6 +17,7 @@ Table of contents:
 * [Development](#development)
   * [Server](#server)
   * [Client](#client)
+* [Data model](#data-model)
 * [Licence](#licence)
 <!-- TOC -->
 
@@ -53,6 +54,13 @@ $ npm run dev
 
 Then go to http://localhost:5173
 
+## Data model
+
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="./assets/data_model_dark.png">
+  <img alt="Data model" src="./assets/data_model.png">
+</picture>
+
 ## Licence
 
 [MIT](./LICENSE)