Add getting started docs

docs/getting-started.md

@@ -0,0 +1,79 @@
+# Getting started with Gomod2nix
+
+## Installation
+
+### Using Niv
+
+First initialize Niv:
+``` bash
+$ niv init --latest
+$ niv add tweag/gomod2nix
+```
+
+Create a `shell.nix` used for development:
+``` nix
+{ pkgs ? (
+    let
+      sources = import ./nix/sources.nix;
+    in
+    import sources.nixpkgs {
+      overlays = [
+        (import "${sources.gomod2nix}/overlay.nix")
+      ];
+    }
+  )
+}:
+
+let
+  goEnv = pkgs.mkGoEnv { pwd = ./.; };
+in
+pkgs.mkShell {
+  packages = [
+    goEnv
+    pkgs.gomod2nix
+    pkgs.niv
+  ];
+}
+```
+
+And a `default.nix` for building your package
+``` nix
+{ pkgs ? (
+    let
+      sources = import ./nix/sources.nix;
+    in
+    import sources.nixpkgs {
+      overlays = [
+        (import "${sources.gomod2nix}/overlay.nix")
+      ];
+    }
+  )
+}:
+
+pkgs.buildGoApplication {
+  pname = "myapp";
+  version = "0.1";
+  pwd = ./.;
+  src = ./.;
+  modules = ./gomod2nix.toml;
+}
+```
+
+### Using Flakes
+
+The quickest way to get started if using Nix Flakes is to use the Flake template:
+``` bash
+$ nix flake init -t github:tweag/gomod2nix#app
+```
+
+## Basic usage
+
+After you have entered your development shell you can generate a `gomod2nix.toml` using:
+``` bash
+$ gomod2nix generate
+```
+
+To speed up development and avoid downloading dependencies again in the Nix store you can import them directly from the Go cache using:
+``` bash
+$ gomod2nix import
+```