docs: add database schema document

Author: morremeyer

docs/database-schema.md

@@ -0,0 +1,19 @@
+# Database schema
+
+This document aims at developers who need to understand how the database schema looks. If you are looking for usage hints, go to [the usage guide](usage.md).
+
+## Entity Relationship diagram
+
+<!-- Use https://mermaid.live for easier editing of this diagram -->
+
+```mermaid
+erDiagram
+    Budget ||--o{ Account : has
+    Budget ||--o{ Category : has
+    Budget ||--o{ Transaction : "has, see #308"
+    Transaction }o..o| Envelope : "has"
+    Transaction }o--|| Account : "has source"
+    Transaction }o--|| Account : "has destination"
+    Category ||--o{ Envelope : has
+    Envelope ||--o{ Allocation : "has max. 1 per Month"
+```