docs: add contributing guidelines

Author: soupglasses

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# CONTRIBUTING.md
+
+This repo follows a modified version of [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/)
+applying the [nixpkgs's contributing guidelines](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md)
+on-top of it.
+
+The following document explains the differences diverging from the standard format.
+
+## Git formatting:
+
+* The commit message **MUST** all be lowercased. The body of the commit has no such restriction.
+* **BREAKING CHANGE**: a commit that introduces a breaking API change (similar to *MAJOR* in Semantic Versioning) **MUST** include a footer in the style of `BREAKING CHANGE: <description>`.
+
+```
+<pkg-name>: (<version> -> <version> | init at <version> | <type? feat> <reason>)
+<module-name>: <type? feat> <reason>
+host/<host-name>: <type? feat> <reason>
+<type>: <reason>
+```
+
+Types:
+
+* **type**: follows conventional commits, `fix`, `ci`, `docs`, `style`, `refactor`, `perf`, `test`, however its optional unless at the top level. Omitting type should be preferred, only add a type if it can explain the change on its own. If not, rephrase the reason to fit the pattern "feat: reason" or "docs: reason", depending on the commit style's default type.
+* **version**: typically semantic version of a package. Follows nixpkgs conventions.
+
+Names:
+
+* *pkg-name*: typically the name of the folder holding a `default.nix` file somewhere under the `packages/` folder.
+* *module-name*: typically the name of the `.nix` file in a `modules/` folder. Never apped the file type. This may also include per-host modules plus overlays and tests relating to supporting a module.
+* *host-name*: typically the name of the folder under `hosts/` folder.
+
+
+## Committing
+
+We use pre-commit. You can check a commit without it using `nix flake check` but this is not recommended.