chore(docs): initial upgrade guide for v2

Author: rubenfonseca

docs/upgrade.md

@@ -0,0 +1,62 @@
+---
+title: Upgrade guide
+description: asdfasdf
+---
+
+<!-- markdownlint-disable MD043 -->
+
+## Migrate to v2 from v1
+
+The transition from Powertools for Python v1 to v2 is as painless as possible, as we strove for minimal breaking changes.
+The API for event handler's Response has minor changes, but we kept the breaking changes to a bare minimum. We've also added some new features to some components.
+
+???+ important
+    Powertools for Python v2 drops suport for Python 3.6, following the Python 3.6 End-Of-Life (EOL) reached on December 23, 2021.
+
+### Initial Steps
+
+Before starting, it is highly suggested to make a copy of your current working project or create a new branch with git.
+
+1. **Upgrade** Python to at least v3.7
+
+2. **Ensure** you have the latest `aws-lambda-powertools`
+
+    ```bash
+    pip install aws-lambda-powertools -U
+    ```
+
+3. **Check** the following sections to see if any of your code is affected
+
+## Event Handler Response (headers and cookies)
+
+The `Response` class of the event handler utility was changed slightly:
+
+1. The `headers` parameter now has a type signature of `Dict[str, List[str]]`
+2. A new `cookies` parameter was added (type `List[str]`)
+
+```python hl_lines="6 12 13"
+@app.get("/todos")
+def get_todos():
+    # Before
+    return Response(
+        # ...
+        headers={"Content-Type": "text/plain"}
+    )
+
+    # After
+    return Response(
+        # ...
+        headers={"Content-Type": ["text/plain"]},
+        cookies=["CookieName=CookieValue"]
+    )
+```
+
+In the same way, it can be more convenient to just append headers to the response object:
+
+```python hl_lines="4 5"
+@app.get("/todos")
+def get_todos():
+    response = Response(...)
+    response.headers["Content-Type"].append("text/plain")
+    response.cookies.append("CookieName=CookieValue")
+```

mkdocs.yml

@@ -10,6 +10,7 @@ nav:
   - Tutorial: tutorial/index.md
   - Roadmap: roadmap.md
   - API reference: api/" target="_blank
+  - Upgrade guide: upgrade.md
   - Core utilities:
       - core/tracer.md
       - core/logger.md