docs: add initial design document

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I99399d520a1339d87072279f76de3cd26a6a6964

docs/DESIGN.md

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+# Design
+
+Notes to self and somewhat of a guide to design some of the design choices
+behind FC. This is not a contribution guideline, and changes to this document
+are welcome if necessary.
+
+## Overview
+
+FC is built as a local replacement for Hydra. Meaning you probably do not want
+to deploy it on your Super Enterprise Friends Group that needs a reliable CI.
+This project is an attempt to utilize our infrastructure members as build
+machines to cache our projects without relying on Github's weak runners.
+
+---
+
+Hydra is the Nix/NixOS project's continuous integration system. It uses Nix to
+declaratively define and build jobs, ensuring reproducible builds. According to
+the NixOS Wiki:
+
+> "Hydra is a tool for continuous integration testing and software release that
+> uses a purely functional language to describe build jobs and their
+> dependencies."
+
+In Hydra:
+
+- A **Project** corresponds to a source repository.
+- A **Jobset** (often per branch or channel) contains many **Jobs** (Nix
+  derivations to build).
+- A `release.nix` ("Release Set") file declares what to build.
+
+Hydra pulls changes from version control, re-evaluates Nix expressions, and
+triggers builds when inputs change.
+
+---
+
+FC commits to this design with minimal tweaks. Most critically, FC is not
+designed to be used alongside Nixpkgs. Sure you can do it, but that is not the
+main goal. The main goal is a distributed, declarative CI that has learned from
+Hydra's mistakes.