Update upgrading docs

Fixes #8181

Author: mjackson

docs/upgrading/v5.md

@@ -25,6 +25,8 @@ In general, the process looks like this:
 
 1. [Upgrade to React v16.8 or greater](#upgrade-to-react-v168)
 2. [Upgrade to React Router v5.1](#upgrade-to-react-router-v51)
+   - [Remove `<Redirect>`s inside `<Switch>`](#remove-directs-inside-switch)
+   - [Refactor custom `<Route>`s](#refactor-custom-routes)
 3. [Upgrade to React Router v6](#upgrade-to-react-router-v6)
 
 The following is a detailed breakdown of each step that should help you migrate quickly and with confidence to v6.
@@ -101,6 +103,36 @@ In summary, to upgrade from v4/5 to v5.1, you should:
 - Replace any `<Route>`s that are not inside a `<Switch>` with `useRouteMatch`,
   or wrap them in a `<Switch>`
 
+### Remove `<Redirect>`s inside `<Switch>`
+
+Remove any `<Redirect>` elements that are directly inside a `<Switch>`.
+
+If you want to redirect on the initial render, you should move the redirect logic to your server (we [wrote more about this here](https://gist.github.com/mjackson/b5748add2795ce7448a366ae8f8ae3bb)).
+
+If you want to redirect client-side, move your `<Redirect>` into a `<Route render>` prop.
+
+```tsx
+// Change this:
+<Switch>
+  <Redirect from="about" to="about-us" />
+</Switch>
+
+// to this:
+<Switch>
+  <Route path="about" render={() => <Redirect to="about-us" />} />
+</Switch>
+```
+
+Normal `<Redirect>` elements that are not inside a `<Switch>` are ok to remain. They will become `<Navigate>` elements in v6.
+
+### Refactor custom `<Route>`s
+
+Replace any elements inside a `<Switch>` that are not plain `<Route>` elements with a regular `<Route>`. This includes any `<PrivateRoute>`-style custom components.
+
+You can [read more about the rationale behind this here](https://gist.github.com/mjackson/d54b40a094277b7afdd6b81f51a0393f), including some tips about how to use a `<Route render>` prop in v5 to achieve the same effect.
+
+### Ship it!
+
 Again, **once your app is upgraded to v5.1 you should test and deploy it**, and pick this guide back up when you're ready to continue.
 
 ## Upgrade to React Router v6