docs: add very basic README

So it doesn't look so empty in here...

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Icff5e32cef6d19e42107f0f4945f26b06a6a6964

README.md

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+# FC
+
+FC is a modern, Rust-based continuous integration system designed to replace
+Hydra for our systems. Heavily work in progress.
+
+## Architecture
+
+- **server**: Web API and UI (Axum)
+- **evaluator**: Git polling and Nix evaluation
+- **queue-runner**: Build dispatch and execution
+- **common**: Shared types and utilities
+
+## Development
+
+```bash
+nix develop
+cargo build
+```
+
+## Components
+
+### Server
+
+Web API server providing REST endpoints for project management, jobsets, and
+build status.
+
+### Evaluator
+
+Periodically polls Git repositories and evaluates Nix expressions to create
+builds.
+
+### Queue Runner
+
+Processes build queue and executes Nix builds on available machines.