Create CONTRIBUTING.md

CONTRIBUTING.md

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+# How do I contribute?
+
+Flake-parts is designed to be extremely modular, so often, you don't have to.
+
+Nonetheless, some changes can only be made here.
+
+Step 1. Look for an open or closed issue. This may be the quickest path to a solution to your problem.
+
+Step 2. If needed, open an issue. This way we can discuss the problem, and if necessary discuss changes, if any need to be made.
+
+Step 3. If needed, create a PR. Make sure to run `nix-shell` before comitting. It installs a pre-commit hook with `nixpkgs-fmt`.
+
+
+# Style
+
+This repository is written in a style similar to that of Nixpkgs, with some exceptions
+
+ - File names may be in camelCase. This reduces the number of unique names in the project.
+
+ - The "contains attribute" operator is spelled without spaces, just like the "select attribute" operator. I believe Nixpkgs is undecided on this.
+   ```nix
+   if x?a then "has a" else "does not have a"
+   #  ^^^
+   ```
+
+Except for file names, the Nixpkgs casing rule is maintained here as well:
+
+ - Package names are verbatim or in snake-case. Example:
+    - `flake-parts-lib`
+
+ - Functionality provided by flake-parts is in camelCase. Examples:
+    - `getSystem`
+    - `mkFlake`