link to popular issues from the contributing guide

this also adds a hint to contributors about making far-reaching changes,
complementing the recent update to the maintainers' handbook on how to
deal with those.

CONTRIBUTING.md

@@ -30,6 +30,9 @@ Check out the [security policy](https://github.com/NixOS/nix/security/policy).
    Issues labeled [good first issue](https://github.com/NixOS/nix/labels/good-first-issue) should be relatively easy to fix and are likely to get merged quickly.
    Pull requests addressing issues labeled [idea approved](https://github.com/NixOS/nix/labels/idea%20approved) or [RFC](https://github.com/NixOS/nix/labels/RFC) are especially welcomed by maintainers and will receive prioritised review.
 
+   If you are proficient with C++, addressing one of the [popular issues](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc) will be highly appreciated by maintainers and Nix users all over the world.
+   For far-reaching changes, please investigate possible blockers and design implications, and coordinate with maintainers before investing too much time in writing code that may not end up getting merged.
+
    If there is no relevant issue yet and you're not sure whether your change is likely to be accepted, [open an issue](https://github.com/NixOS/nix/issues/new/choose) yourself.
 
 2. Check for [pull requests](https://github.com/NixOS/nix/pulls) that might already cover the contribution you are about to make.