Add basic contributing guide

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

README.md

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+# Nix Dotfiles
+
+This repository contains the flake required to build critical and personal
+infrastructure running NixOS. The setup can be explored as follows.
+
+This repo supports `x86_64-linux` and `aarch64-linux`, and in theory supports
+Mac (althought that hasn't been tested).
+
+## Setting Up
+
+Please see [our setup guide](./docs/setting-up.md) for more information on how
+to onboard a new user or system.
+
+## Interested?
+
+Although we are not actively looking for new members to join in on this repo,
+we are not strictly opposed. Please reach out to @ahuston-0 or @RichieCahill
+for further information.
+
+## Repo Structure
+
+- `keys/`: PGP public keys, for those who are using `SOPS` for secrets
+- `modules/`: Nix modules created by us for common services or overrides
+(fail2ban, hydra, certain boot params, etc.)
+- `systems/`: config for common *server* components, as well as per-server configurations
+- `users/`: this directory has two major subdirectories, both are critical
+  - `users/<user>`: includes configs for `home-manager`, `SOPS`, and `SSH` keys
+  , for those who use them
+  - `users/<user>/systems`: functions similarly to `systems/`, although for
+  laptops, desktops, and even a Raspberry Pi now
+
+## Contributing
+
+For members of our organization who are looking to either contribute to the
+existing infrastructure, or onboard their own hardware, please see
+[our contributors guide](./docs/CONTRIBUTING.md)