CONTRIBUTING.md: Move to repo root, where it is more visible.

We found that many users found it difficult to locate this document.

Github supports it in the root, see:
https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

README.md

@@ -87,7 +87,7 @@ Most contributions are based on and merged into these branches:
   deemed of sufficiently high quality
 
 For more information about contributing to the project, please visit
-the [contributing page](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).
+the [contributing page](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md).
 
 # Donations