Add new file

osa0/0.6.md

+```mermaid
+sequenceDiagram
+    participant browser
+    participant server
+    
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/notes
+    activate server
+    server-->>browser: HTML document
+    deactivate server
+    
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/main.css
+    activate server
+    server-->>browser: the css file
+    deactivate server
+    
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/main.js
+    activate server
+    server-->>browser: the JavaScript file
+    deactivate server
+    
+    Note right of browser: The browser starts executing the JavaScript code that fetches the JSON from the server
+    
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/data.json
+    activate server
+    server-->>browser: [{ "content": "HTML is easy", "date": "2023-1-1" }, ... ]
+    deactivate server    
+
+    Note right of browser: The browser executes the callback function that renders the notes 
+    ```