Merge pull request rust-lang#2481 from xizheyin/submit-in-rdg-or-r

Explain where rdg changes should be submitted

Author: tshepang

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

@@ -434,6 +434,10 @@ 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
+
+For detailed information about where to contribute rustc-dev-guide changes and the benefits of doing so, see [the rustc-dev-guide working group documentation](https://forge.rust-lang.org/wg-rustc-dev-guide/index.html#where-to-contribute-rustc-dev-guide-changes).
+
 ## Issue triage
 
 Please see <https://forge.rust-lang.org/release/issue-triaging.html>.