Add README file

LICENSE

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+MIT License
+
+Copyright (c) 2019 Tweag I/O
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# Gomod2nix
+Convert applications using Go modules -> Nix
+
+## Usage
+From the Go project directory execute:
+``` bash
+$ gomod2nix
+```
+
+This will create `gomod2nix.toml` that's used like so
+``` nix
+let
+  pkgs = import <nixpkgs> {
+    overlays = [
+      (self: super: {
+        buildGoApplication = super.callPackage ./builder { };
+      })
+    ];
+  };
+in pkgs.buildGoApplication {
+  pname = "gomod2nix-example";
+  version = "0.1";
+  src = ./.;
+  modules = ./gomod2nix.toml;
+}
+```
+
+## FAQ
+
+### Why not continue work on vgo2nix?
+Vgo2nix was built on top of the old Nixpkgs build abstraction `buildGoPackage`, this abstraction was built pre-modules and suffered from some fundamental design issues with modules, such as only allowing a single version of a Go package path inside the same build closure, something that Go itself allows for.
+
+We need a better build abstraction that takes Go modules into account, while remaining [import from derivation](https://nixos.wiki/wiki/Import_From_Derivation)-free.
+
+### Will this be included in Nixpkgs
+
+Yes. Once the API is considered stable.
+
+## License
+
+This project is licensed under the MIT License. See the [LICENSE](LICENSE)
+file for details.