README: use relative links to CONTRIBUTING.md

There’s two different ways of viewing rendered versions of the Markdown
files in this repo. You can either

1. view them on GitHub or
2. clone the repo and open them using a program like Okular [1].

Before this change, option 2 was a little bit inconvenient. When you
clicked on either of the links to CONTRIBUTING.md, it would take you to
GitHub instead of opening up the Markdown file locally.

[1]: <https://okular.kde.org/>

README.md

@@ -8,7 +8,7 @@
 </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>
 
@@ -73,7 +73,7 @@ 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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