Bring back the breaking changes readme section (#2804)

axum/README.md

@@ -23,6 +23,12 @@ In particular the last point is what sets `axum` apart from other frameworks.
 authorization, and more, for free. It also enables you to share middleware with
 applications written using [`hyper`] or [`tonic`].
 
+## Breaking changes
+
+We are currently working towards axum 0.8 so the `main` branch contains breaking
+changes. See the [`0.7.x`] branch for what's released to crates.io and up to
+date changelogs.
+
 ## Usage example
 
 ```rust