rustc-dev-guide subtree update

src/doc/rustc-dev-guide/README.md

@@ -91,6 +91,16 @@ Older versions of `josh-proxy` may not round trip commits losslessly so it is im
 3) Push the branch to your fork and create a PR into `rustc-dev-guide`
 
 ### Push changes from this repository into `rust-lang/rust`
+
+NOTE: If you use Git protocol to push to your fork of `rust-lang/rust`,
+ensure that you have this entry in your Git config,
+else the 2 steps that follow would prompt for a username and password:
+
+```
+[url "[email protected]:"]
+insteadOf = "https://github.com/"
+```
+
 1) Run the push command to create a branch named `<branch-name>` in a `rustc` fork under the `<gh-username>` account
     ```
     cargo run --manifest-path josh-sync/Cargo.toml rustc-push <branch-name> <gh-username>