add RLS note

Author: pietroalbini

posts/2022-09-22-Rust-1.64.0.md

@@ -192,6 +192,10 @@ Rust 1.64 includes some changes that might prevent you from upgrading:
    versions, which have been out for more than a year. The vast majority of
    impacted users should be able to mitigate with a `cargo update`.
 
+As part of the [RLS
+deprecation](https://blog.rust-lang.org/2022/07/01/RLS-deprecation.html), this
+is also the last release containing a copy of RLS. Starting from Rust 1.65.0,
+RLS will be replaced by a small LSP server showing the deprecation warning.
 
 ### Other changes