Document breaking changes related to nest (#1684)

axum/CHANGELOG.md

@@ -158,6 +158,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   async fn fallback() {}
   ```
 
+- **breaking:** It is no longer supported to `nest` twice at the same path, i.e.
+  `.nest("/foo", a).nest("/foo", b)` will panic. Instead use `.nest("/foo", a.merge(b))`
+- **breaking:** It is no longer supported to `nest` a router and add a route at
+  the same path, such as `.nest("/a", _).route("/a", _)`. Instead use
+  `.nest("/a/", _).route("/a", _)`.
 - **changed:** `Router::nest` now only accepts `Router`s, the general-purpose
   `Service` nesting method has been renamed to `nest_service` ([#1368])
 - **breaking:** Allow `Error: Into<Infallible>` for `Route::{layer, route_layer}` ([#924])