Add usage section for session_token in README

Author: NabeelAhsen

README.md

@@ -120,6 +120,60 @@ _Note: Your application must be public to test the billing process. To test on a
     has_been_billed = activated_charge.status == 'active'
     ```
 
+### Session tokens
+
+The Shopify Python API library provides helper methods to decode [session tokens](https://shopify.dev/concepts/apps/building-embedded-apps-using-session-tokens). You can use the `get_decoded_session_token` function to help extract and decode a session token from an HTTP Authorization header.
+
+#### Basic usage
+
+```python
+from shopify import session_token
+
+decoded_payload = session_token.get_decoded_session_token(
+    authorization_header=your_auth_request_header,
+    api_key=your_api_key,
+    secret=your_api_secret,
+)
+```
+
+#### Use session_token to create a decorator
+
+Here's a sample use-case of building a Django decorator using `session_token`:
+
+```python
+from shopify import session_token
+
+
+def session_token_required(func):
+    def wrapper(*args, **kwargs):
+        request = args[0]  # Or flask.request if you use Flask
+        try:
+            session_token = session_token.get_decoded_session_token(
+                authorization_header = request.headers.get('Authorization'),
+                api_key = SHOPIFY_API_KEY,
+                secret = SHOPIFY_API_SECRET
+            )
+            with shopify_session(session_token):
+                return func(*args, **kwargs)
+        except session_token.SessionTokenError:
+            return generate_http_401_response()
+
+    return wrapper
+
+
+def shopify_session(session_token):
+    shopify_domain = session_token.get("dest")
+    access_token = get_offline_access_token_by_shop_domain(shopify_domain)
+
+    return shopify.Session.temp(shopify_domain, SHOPIFY_API_VERSION, access_token)
+
+
+@session_token_required
+def products(request):
+    products = shopify.Product.find()
+    ...
+```
+
 ### Advanced Usage
 It is recommended to have at least a basic grasp on the principles of the [pyactiveresource](https://github.com/Shopify/pyactiveresource) library, which is a port of rails/ActiveResource to Python and upon which this package relies heavily.