Add new translation file for rules in musdb

backend/de/rules.json

@@ -0,0 +1,7 @@
+{
+    "tlRules": {
+        "rule_added": "The new rule description has been added",
+        "rule_removed": "The rule description has been removed",
+        "rule_updated": "The rule has been updated"
+    }
+}