CONTRIBUTING: expand contributor and reviewer guidelines

Document the full contribution workflow including feature branches,
README/flake.nix entry formats, testing from forks, deprecation
process, and update reviewer section to use GitHub merge queues.

Also clarify that any nixpkgs committer can review PRs.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,5 @@
+<!-- Please read the CONTRIBUTING.md guidelines before submitting: https://github.com/NixOS/nixos-hardware/blob/master/CONTRIBUTING.md -->
+
 ###### Description of changes
 
 

CONTRIBUTING.md

@@ -1,5 +1,11 @@
 # Contributing a Device Profile
 
+Please submit your contributions from a feature branch, not from your master branch:
+
+```bash
+git checkout -b my-new-hardware-profile
+```
+
 ## 1. Writing profiles
 
 Create an appropriate directory and start writing your expression.
@@ -36,18 +42,144 @@ should either be conservative or be guarded behind additional NixOS module
 options. If it makes sense to have a performance-focussed config, it can be
 declared in a separate profile.
 
-## 2. Adding Entry
+## 2. Optional: Add a README
 
-Link the profile in the table in README.md and in flake.nix.
+Profiles can include a `README.md` with device-specific documentation such as:
+- Firmware update instructions
+- BIOS settings
+- Known issues and workarounds
+- Links to external resources
 
-## 3. Testing
+See [framework/13-inch/7040-amd/README.md](framework/13-inch/7040-amd/README.md)
+for a good example.
 
-Run `nix run .#run-tests` to evaluate all hardware profiles.
+## 3. Adding Entry
+
+Add your profile to both `README.md` and `flake.nix`.
+
+**README.md**: Add a row to the table in alphabetical order by manufacturer:
+
+```markdown
+| [Manufacturer Model](manufacturer/model)    | `<nixos-hardware/manufacturer/model>`    | `manufacturer-model`    |
+```
+
+The columns are:
+1. Model name linking to the profile directory
+2. Channel path (for `imports = [ <nixos-hardware/...> ]`)
+3. Flake module name (for `nixos-hardware.nixosModules.<name>`)
+
+**flake.nix**: Add an entry to the `nixosModules` attribute set:
+
+```nix
+manufacturer-model = import ./manufacturer/model;
+```
+
+The flake module name should use hyphens (e.g., `dell-xps-15-9500`).
+
+## 4. Multiple Variants
+
+For devices with multiple configurations (e.g., different GPU options), create
+subdirectories with a shared base module. For example:
+
+```
+dell/xps/15-9560/
+├── default.nix          # Base profile (or one default variant)
+├── intel/default.nix    # Intel-only variant
+├── nvidia/default.nix   # Nvidia variant
+└── xps-common.nix       # Shared configuration
+```
+
+Each variant needs its own entry in README.md and flake.nix:
+
+```nix
+dell-xps-15-9560 = import ./dell/xps/15-9560;
+dell-xps-15-9560-intel = import ./dell/xps/15-9560/intel;
+dell-xps-15-9560-nvidia = import ./dell/xps/15-9560/nvidia;
+```
+
+## 5. Testing
+
+Before opening a pull request, test your profile by pointing to your fork.
+
+**Using channels:**
+
+```bash
+sudo nix-channel --add https://github.com/<github-user>/nixos-hardware/archive/<branch>.tar.gz nixos-hardware
+sudo nix-channel --update
+sudo nixos-rebuild test
+```
+
+**Using flakes:**
+
+Update your `flake.nix` input temporarily:
+
+```nix
+inputs.nixos-hardware.url = "github:<github-user>/nixos-hardware/<branch>";
+```
+
+Then run:
+
+```bash
+sudo nixos-rebuild test --flake .
+```
+
+You can also run `nix run .#run-tests` to evaluate all hardware profiles.
 Because profiles can only be tested with the appropriate hardware, quality
 assurance is up to *you*.
 
+## Compatibility
+
+Profiles should work with both NixOS unstable and the current stable release.
+
+## Deprecating Modules
+
+When renaming or removing a module, add deprecation warnings for both flake and
+channel users.
+
+**For flake users**, use the `deprecated` helper in `flake.nix`:
+
+```nix
+old-module-name = deprecated "issue-number" "old-module-name" (import ./new/path);
+```
+
+**For channel users**, add an assertion in the old module's `default.nix`:
+
+```nix
+{ ... }:
+
+{
+  assertions = [
+    {
+      assertion = false;
+      message = "Importing path/to/old/module (default.nix) directly is deprecated! See https://github.com/NixOS/nixos-hardware/blob/master/path/to/DEPRECATION.md for more details";
+    }
+  ];
+}
+```
+
+Include a markdown file explaining the migration path. See
+[asus/zephyrus/ga402x/ATTR-SET-DEPRECATION.md](asus/zephyrus/ga402x/ATTR-SET-DEPRECATION.md)
+for an example.
+
+## Getting Help
+
+For questions and discussions, join us in the
+[#nixos-hardware:nixos.org](https://matrix.to/#/#nixos-hardware:nixos.org)
+Matrix channel.
+
 # For reviewers:
 
-This repository has [mergify](https://mergify.com/) enabled for easier merging after a successfull build:
+Any nixpkgs committer can review and approve pull requests in this repository.
 
-* `@mergify queue` will add the current pull request to the merge queue and merge when all tests succeed
+This repository uses GitHub merge queues for merging pull requests.
+
+## Review checklist
+
+1. **README.md entry**: Verify the contribution adds the device to the table in README.md
+2. **flake.nix entry**: Verify the profile is exposed in flake.nix
+3. **Common profiles**: Since testing on real hardware isn't always possible,
+   check that the profile re-uses appropriate common profiles (e.g., cpu, gpu,
+   laptop/pc, ssd/hdd modules)
+4. **Previous contributors**: For changes to existing profiles, consider
+   requesting feedback from previous contributors who may have access to the
+   hardware