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

.github/PULL_REQUEST_TEMPLATE.md

@@ -23,8 +23,8 @@ Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-
 - [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
 - [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nixpkgs-review --run "nixpkgs-review wip"`
 - [ ] Tested execution of all binary files (usually in `./result/bin/`)
-- [21.11 Release Notes (or backporting 21.05 Relase notes)](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md#generating-2111-release-notes)
+- [21.11 Release Notes (or backporting 21.05 Relase notes)](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#generating-2111-release-notes)
   - [ ] (Package updates) Added a release notes entry if the change is major or breaking
   - [ ] (Module updates) Added a release notes entry if the change is significant
   - [ ] (Module addition) Added a release notes entry if adding a new NixOS module
-- [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).
+- [ ] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md).

.github/workflows/backport.yml

@@ -25,5 +25,5 @@ jobs:
           pull_description: |-
             Bot-based backport to `${target_branch}`, triggered by a label in #${pull_number}.
 
-            * [ ] Before merging, ensure that this backport complies with the [Criteria for Backporting](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md#criteria-for-backporting-changes).
+            * [ ] Before merging, ensure that this backport complies with the [Criteria for Backporting](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#criteria-for-backporting-changes).
               * Even as a non-commiter, if you find that it does not comply, leave a comment.

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
 

doc/contributing/submitting-changes.chapter.md

@@ -62,7 +62,7 @@
 
 - Push your changes to your fork of nixpkgs.
 - Create the pull request
-- Follow [the contribution guidelines](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md#submitting-changes).
+- Follow [the contribution guidelines](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#submitting-changes).
 
 ## Submitting security fixes {#submitting-changes-submitting-security-fixes}
 
@@ -193,7 +193,7 @@ It’s important to test any executables generated by a build when you change or
 
 ### Meets Nixpkgs contribution standards {#submitting-changes-contribution-standards}
 
-The last checkbox is fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md). The contributing document has detailed information on standards the Nix community has for commit messages, reviews, licensing of contributions you make to the project, etc\... Everyone should read and understand the standards the community has for contributing before submitting a pull request.
+The last checkbox is fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md). The contributing document has detailed information on standards the Nix community has for commit messages, reviews, licensing of contributions you make to the project, etc\... Everyone should read and understand the standards the community has for contributing before submitting a pull request.
 
 ## Hotfixing pull requests {#submitting-changes-hotfixing-pull-requests}