adding docs subtree

docs/step2-09/README.md

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+# Step 2.9
+
+[Lessons](../) | [Exercise](./exercise/) | [Demo](./demo/)
+
+Redux Thunk middleware for actions with service calls. The documentation is here:
+
+https://github.com/reduxjs/redux-thunk
+
+Action creators are a natural place to put service calls. Redux thunk middleware passes in the `dispatch()` and `getState()` from the store into the action creators. This allows the action creator itself to dispatch different actions in between async side effects. Combined with the async / await syntax, coding service calls is a cinch!
+
+Most of the time, in a single-page app, we apply **optimistic UI updates**. We can update the UI before the network call completes so the UI feels more responsive.
+
+# Exercise
+
+1. open up `exercise/src/service/index.ts` and study the signature of the functions to call the service such as the `add()` function
+
+2. open `exercise/src/actions/index.ts` and fill in the missing content inside `actionsWithService`
+
+- note that the `complete` and `clear` functions require you to write your own wrapper function
+
+3. open `exercise/src/index.tsx` and follow the instructions in the TODO comment to make the app prepopulate with data from the service.