Updated datamodel picture in the readme

Reviewed-on: #6
Co-authored-by: Jarno Kiesiläinen <jarnokie@gmail.com>
Co-committed-by: Jarno Kiesiläinen <jarnokie@gmail.com>

README.md

@@ -5,10 +5,28 @@
 ```mermaid
 classDiagram
     class Activity {
+        +Long id
         +String name
         +Location location
-        +Int priceRange
+        +Integer priceRange
         +List~String~ tags
+        +String description
+        +LocalDateTime createdAt
+        +LocalDateTime updatedAt
     }
+    
+    class Location {
+        +Long id
+        +String name
+        +String address
+        +String city
+        +String country
+        +String postalCode
+        +Double latitude
+        +Double longitude
+        +LocalDateTime createdAt
+        +LocalDateTime updatedAt
+    }
+    
+    Activity <-- Location : belongs to
 ```
-