Add a reference to the nativelink benchmarks repository

Author: skyrpex

CONTRIBUTING.md

@@ -478,3 +478,9 @@ nix run .#nativelinkCoverageForHost
 ```
 
 The `result` symlink contains a webpage with the visualized report.
+
+## Independent benchmarks
+
+The third-party repository [skyrpex/nativelink-benchmarks](https://github.com/skyrpex/nativelink-benchmarks) is a project that gathers benchmarking data for every commit in NativeLink.
+
+You can view the benchmark data evolution over time at [https://nativelink-benchmarks.vercel.app/](https://nativelink-benchmarks.vercel.app/).