Add README

Signed-off-by: Marcel Müller <neikos@neikos.email>

README.md

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+# Dylint as a nix library
+
+This repository contains a nix library that allows easy access to the
+[dylint](https://github.com/trailofbits/dylint) project in a reproducible way.
+
+
+## How to use
+
+Dylint has some [great
+documentation](https://github.com/trailofbits/dylint/blob/f4cdda1d071022a2bb846b810333544d1238559b/docs/2024-10-11%20Linting%20with%20Dylint%20(EuroRust).pdf)
+on how Dylint itself works.
+
+This library then compiles and assembles the different parts you need to get it
+working inside nix.
+
+### Prerequisites
+
+Using `nix-dylint` requires the following:
+
+- A `nixpkgs` instance with the `rust-overlay` applied from
+  [`oxalica/rust-overlay`](https://github.com/oxalica/rust-overlay/tree/master).
+- A list of lints and their nightly toolchains, formatted as e.g.
+  `nightly-{date}`
+
+### Building a dylint instance
+
+Dylint tries its best to do everything for you, but this clashes with the way
+`nix`-users generally use their software. So this library generates a
+`cargo-dylint` wrapper with the relevant env variables set such that it should
+not build anything.
+
+Assembling it all, it might look like this:
+
+```nix
+    dylintLib = inputs.nix-dylint.mkLib { inherit pkgs crane; };
+    dylint = dylintLib.mkDylint { lints = [
+        {
+            toolchain = "nightly-2025-01-09";
+            package = dylintLib.cargo-dylint-general;
+        }
+    ]; }
+```
+
+
+`dylint` is then a derivation that provides the `cargo-dylint` executable. One
+can then use it as usual.