Add upgrade docs.

docs/upgrading.md

@@ -1,5 +1,11 @@
 # Upgrading
 
+## Upgrade to v9
+
+The rejection value for failed requests with `useFetch` was changed. Previously it was the Response object. Now it's an
+Error object with `response` property. If you are using `useFetch` and are using the `error` value, expecting it to be
+of type Response, you must now use `error.response` instead.
+
 ## Upgrade to v8
 
 All standalone helper components were renamed to avoid import naming collision.
@@ -12,7 +18,8 @@ All standalone helper components were renamed to avoid import naming collision.
 
 > A [codemod](https://github.com/async-library/react-async/tree/master/codemods) is available to automate the upgrade.
 
-The return type for `run` was changed from `Promise` to `undefined`. You should now use the `promise` prop instead. This is a manual upgrade. See [`promise`](installation.md#promise-1) for details.
+The return type for `run` was changed from `Promise` to `undefined`. You should now use the `promise` prop instead. This
+is a manual upgrade. See [`promise`](state.md#promise) for details.
 
 ## Upgrade to v6
 
@@ -25,11 +32,8 @@ The return type for `run` was changed from `Promise` to `undefined`. You should
 ## Upgrade to v4
 
 - `deferFn` now receives an `args` array as the first argument, instead of arguments to `run` being spread at the front
-
   of the arguments list. This enables better interop with TypeScript. You can use destructuring to keep using your
-
   existing variables.
 
 - The shorthand version of `useAsync` now takes the `options` object as optional second argument. This used to be
-
   `initialValue`, but was undocumented and inflexible.