[HW-1] added docs

docs/homework-1/img/cart-cart-item-add.plantuml

@@ -0,0 +1,21 @@
+@startuml
+
+actor User as u
+collections Cart as c
+database CartStorage as cs
+collections ProductService as p
+
+u -> c : POST /user/<user_id>/cart/<sku_id>\n\t- count
+activate c
+
+c -> p : GET /product/<sku>
+activate p
+  p -> c : \nResponse: 200 OK\n\t- name\n\t- price\n\t- sku
+deactivate p
+c -> c : validate product exists
+c -> cs : cart.AddItem()
+c -> u : Response: 200 OK
+
+deactivate c
+
+@enduml