message: docs: add migration notes
diff: @@ -20,13 +20,23 @@ Since the library is still unstable **and because we want feedback** on bugs and
  .map(record => Object.values(record.components))
  .flat()
  ```
  -- If you use a `transition`, you need to wait for the router to be _ready_ before mounting the app:
  +- If you use a `transition`, you may need to wait for the router to be _ready_ before mounting the app:
  ```js
  app.use(router)
  // Note: on Server Side, you need to manually push the initial location
  router.isReady().then(() => app.mount('#app'))
  ```
  - Otherwise there will be an initial transition as if you provided the `appear` prop to `transition` because the router displays its initial location (nothing)
  + Otherwise there will be an initial transition as if you provided the `appear` prop to `transition` because the router displays its initial location (nothing) and then displays the first location. This happens because navigations are all asynchronous now. **If you have navigation guards upon the initial navigation**, you might not want to block the app render until they are resolved.
  +- On SSR, you need to manually pass the appropriate history by using a ternary:
  + ```js
  + let history = isServer ? createMemoryHistory() : createWebHistory()
  + let router = createRouter({ routes, history })
  + // on server only
  + router.push(req.url) // request url
  + router.isReady().then(() => {
  + // resolve the request
  + })
  + ```
  #### Improvements
  @@ -36,8 +46,16 @@ These are technically breaking changes but they fix an inconsistent behavior.
  - _resolving_(`router.resolve`) or _pushing_ (`router.push`) a location with missing params no longer warns and produces an invalid URL (`/`), but explicitly throws an Error instead.
  - Empty children `path` does not append a trailing slash (`/`) anymore to make it consistent across all routes:
  - By default no route has a trailing slash but also works with a trailing slash
  - - Adding `strict: true` to a route record or `pathOptions: { strict: true }` to the router options (alongside `routes`) will disallow an optional trailing slash
  - - Combining the point above with a trailing slash in your routes allows you to enforce a trailing slash in your routes
  + - Adding `strict: true` to a route record or to the router options (alongside `routes`) will disallow an optional trailing slash
  + - Combining the point above with a trailing slash in your routes allows you to enforce a trailing slash in your routes. In the case of nested routes, make sure to add the trailing slash to the parent:
  + ```js
  + let routes = [
  + {
  + path: '/parent/',
  + children: [{ path: '' }, { path: 'child1/' }, { path: 'child2/' }],
  + },
  + ]
  + ```
  - To redirect the user to trailing slash routes (or the opposite), you can setup a `beforeEach` navigation guard that ensures the presence of a trailing slash
  - Because of the change above, relative children path `redirect` on an empty path are not supported anymore. Use named routes instead:
  ```js
  