add: note about copy from rust-forge

src/doc/rustc-dev-guide/src/compiler-debugging.md

@@ -3,6 +3,10 @@ layout: default
 title: Debugging the Compiler
 ---
 
+**Note: This is copied from the 
+[rust-forge](https://github.com/rust-lang-nursery/rust-forge). If anything needs
+ updating, please open an issue or make a PR on the github repo.**
+
 # Debugging the compiler
 [debugging]: #debugging