Update readme with link to breaking changes page of website. (#69427)

README.md

diff --git a/README.md b/README.md
index 3cff479..95622e7 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,8 @@
 
 For announcements about new releases and breaking changes, follow the
 [flutter-announce@googlegroups.com](https://groups.google.com/forum/#!forum/flutter-announce)
-mailing list.
+mailing list or see the
+[breaking changes](https://flutter.dev/docs/release/breaking-changes) page.
 
 ## About Flutter