Update 'README.md' for DCO

... using the same wording as in 'CONTRIBUTING.md'.

Author: tschwinge

README.md

@@ -193,7 +193,7 @@ To emit Rust front end debug output, you may add options like `-frust-debug`, `-
 If you want to contribute to GCC Rust, you can find more information in [CONTRIBUTING.md](https://github.com/Rust-GCC/gccrs/blob/master/CONTRIBUTING.md).
 
 Please be aware this project is designed to be pushed upstream to GCC when we reach some milestones, and this means we require
-contributions to have copyright assignment in place. Please see https://gcc.gnu.org/contribute.html.
+copyright assignment or the Developer's Certificate of Origin sign-off. Please see the [Contributing to GCC](https://gcc.gnu.org/contribute.html) guide or [Developer's Certificate of Origin (DCO) Sign-off](https://gcc.gnu.org/dco.html) guide.
 
 Not all contributions must be code; we would love to see new test cases or bugs and issues to be reported. Feel free to add any comments on open PRs