Update README.md

Co-authored-by: Boxy <[email protected]>

Author: jieyouxu

README.md

@@ -79,6 +79,7 @@ You'll need to install `josh-proxy` locally via
 ```
 cargo +stable install josh-proxy --git https://github.com/josh-project/josh --tag r24.10.04
 ```
+Older versions of `josh-proxy` may not round trip commits losslessly so it is important to install this exact version.
 
 ### Pull changes from `rust-lang/rust` into this repository
 1) Checkout a new branch that will be used to create a PR into `rust-lang/rustc-dev-guide`