Explain where rdg changes should be submitted

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

src/contributing.md

@@ -434,6 +434,16 @@ Just a few things to keep in mind:
   it might benefit from having a Table of Contents at the beginning,
   which you can auto-generate by including the `<!-- toc -->` marker at the top.
 
+#### ⚠️ Note: Where to contribute `rustc-dev-guide` changes
+
+If your change **only involves the documentation content of rustc-dev-guide** and **does not accompany `rust-lang/rust` code changes**,
+please submit your changes and PRs directly to the [rust-lang/rustc-dev-guide](https://github.com/rust-lang/rustc-dev-guide) repository.
+
+There are some benifits following this rule:
+- Changes to `rustc-dev-guide` repo can be immediately reflected in [the live rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/).
+- Changes to `rustc-dev-guide` repo do not need to go through bors CI in `rust-lang/rust`.
+- Less burden on bors queue in `rust-lang/rust`.
+
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