README: use relative links to CONTRIBUTING.md (#211584)

README.md

@@ -9,7 +9,7 @@
 </p>
 
 <p align="center">
-  <a href="https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md"><img src="https://img.shields.io/github/contributors-anon/NixOS/nixpkgs" alt="Contributors badge" /></a>
+  <a href="CONTRIBUTING.md"><img src="https://img.shields.io/github/contributors-anon/NixOS/nixpkgs" alt="Contributors badge" /></a>
   <a href="https://opencollective.com/nixos"><img src="https://opencollective.com/nixos/tiers/supporter/badge.svg?label=supporters&color=brightgreen" alt="Open Collective supporters" /></a>
 </p>
 
@@ -74,7 +74,7 @@ Community contributions are always welcome through GitHub Issues and
 Pull Requests.
 
 For more information about contributing to the project, please visit
-the [contributing page](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md).
+the [contributing page](CONTRIBUTING.md).
 
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