Add README

README.md

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+# vs-overlay
+
+A collection of packages (mostly plugins) related to using VapourSynth with Nix.
+
+### Standalone Nix
+
+Add the repository to `~/.config/nixpkgs/overlays.nix`:
+
+```nix
+[
+  (import (builtins.fetchTarball "https://github.com/tadeokondrak/vs-overlay/archive/master.tar.gz"))
+]
+```
+
+### NixOS, without flakes
+
+Add the repository to the `nixpkgs.overlays` option of `configuration.nix`:
+
+```nix
+{
+  nixpkgs.overlays = [
+    (import (builtins.fetchTarball "https://github.com/tadeokondrak/vs-overlay/archive/master.tar.gz"))
+  ];
+}
+```
+
+### NixOS, with flakes
+
+> Note: Flakes are an experimental feature of Nix.  
+> If you're not already using them, you should follow [NixOS, without flakes](#nixos-without-flakes) instead.
+
+> Note: This only fetches the overlay as a flake,
+> it doesn't use the pinned Nixpkgs version.  
+> See [Usage as a flake](#usage-as-a-flake) for more information.
+
+In `flake.nix`, add `vs-overlay` as an input:
+
+```nix
+{
+  inputs.vs-overlay.url = "github:tadeokondrak/vs-overlay";
+}
+```
+
+Then add `vs-overlay.overlay` to `nixpkgs.overlays`:
+
+```nix
+{
+  nixpkgs.overlays = [
+    vs-overlay.overlay
+  ];
+}
+```
+
+## Usage
+
+Adding the repository as an overlay will add the packages to the main `pkgs` set,
+using them is the same as packages from Nixpkgs:
+
+### Manually building
+
+```sh
+# Build a standalone package
+nix-build '<nixpkgs>' -A getnative
+
+# Build a VSEdit package with the given plugins
+nix-build -E '
+  { pkgs ? import <nixpkgs> {} }:
+  with pkgs;
+  vapoursynth-editor.withPlugins [
+    vapoursynthPlugins.mvtools
+    vapoursynthPlugins.vsutil
+  ]
+'
+```
+
+### NixOS config
+
+```nix
+{
+  environment.systemPackages = with pkgs; [
+    # Standalone package from this overlay
+    getnative
+
+    # Vapoursynth (from Nixpkgs) with plugins
+    (vapoursynth.withPlugins [
+        vapoursynthPlugins.mvtools
+    ])
+
+    # VSEdit (from Nixpkgs) with plugins
+    (vapoursynth-editor.withPlugins [
+        vapoursynthPlugins.vsutil
+    ])
+  ];
+}
+```
+
+## Usage as a flake
+
+> Note: [Flakes](https://nixos.wiki/wiki/Flakes) are an experimental Nix feature intended to improve reproducibility.
+>
+> When this repository is used as a flake,
+> it uses a pinned version of Nixpkgs rather than the system set.  
+> This has both advantages and disadvantages:
+> a newer revision of Nixpkgs could have an important fix for a plugin dependency,
+> but an update could also break a plugin due to backwards-incompatible changes.
+
+### Setup
+
+In `flake.nix`, add `vs-overlay` as an input:
+
+```nix
+{
+  inputs.vs-overlay.url = "github:tadeokondrak/vs-overlay";
+}
+```
+
+### Usage
+
+Packages are accessible through `vs-overlay.packages.<system>`:
+
+```nix
+{
+  environment.systemPackages = with pkgs; [
+    vs-overlay.packages.x86_64-linux.getnative
+
+    (vapoursynth.withPlugins [
+      vs-overlay.packages.x86_64-linux.vapoursynthPlugins.vsutil
+    ])
+  ];
+}
+```
+
+# License
+
+Like Nixpkgs itself, this repository uses the MIT license.  
+It only applies to the files in this repository,
+not the results of building the packages.