Reuse "Before hacking on Cargo" to emphasize discussion before hacking

CONTRIBUTING.md

@@ -3,3 +3,14 @@
 Contributing documentation has moved to the **[Cargo Contributor Guide]**.
 
 [Cargo Contributor Guide]: https://rust-lang.github.io/cargo/contrib/
+
+## Before hacking on Cargo
+
+We encourage people to discuss their design before hacking on code. Typically,
+you [file an issue] or start a thread on the [internals forum] before submitting
+a pull request. Please read [the process] of how features and bugs are managed
+in Cargo.
+
+[internals forum]: https://internals.rust-lang.org/c/tools-and-infrastructure/cargo
+[file an issue]: https://github.com/rust-lang/cargo/issues
+[the process]: https://doc.crates.io/contrib/process/index.html