Add a shell.nix and instructions for using it

CONTRIBUTING.md

@@ -97,7 +97,9 @@ git clone git@github.com:typelevel/cats.git
 
 To build Cats you should have
 [sbt](http://www.scala-sbt.org/0.13/tutorial/Setup.html) and [Node.js](https://nodejs.org/)
- installed. Run `sbt`, and then use any of the following commands:
+ installed. If you'd like, you can use the [Nix Cats development environment](#nix-cats-development-environment).
+
+ Run `sbt`, and then use any of the following commands:
 
  * `compile`: compile the code
  * `console`: launch a REPL
@@ -202,6 +204,8 @@ run `sbt docs/makeMicrosite`
 
     `gem install jekyll`
 
+    Or just dropping into a `nix-shell` if you are using the [Nix Cats development environment](#nix-cats-development-environment).
+
 2. In a shell, navigate to the generated site directory in `docs/target/site`
 
 3. Start jekyll with `jekyll serve`
@@ -278,3 +282,14 @@ escalate into larger problems.
 
 If you are being harassed, please contact one of [us](https://github.com/typelevel/cats#maintainers)
 immediately so that we can support you.
+
+## Nix Cats Development Environment
+
+Since Cats development can include the Scala runtime, the Scala.js runtime, the Cats website, and more; a number of dependencies (SBT, Node.js, Jekyll, etc) can be needed to work on Cats. Managing these dependencies globally can be a hassle and can lead to version conflicts. To make this easier to manage in an isolated development environment, Cats provides a `shell.nix` for anyone using the [Nix package manager](https://nixos.org/nix/).
+
+To use the Nix-based Cats development environment:
+
+1. [Install](https://nixos.org/nix/download.html) the Nix package manager.
+2. At the root level of the Cats repository, run `nix-shell --pure`. This will drop you into a minimal bash shell that has just the required dependencies on the `PATH`. Note that the first time that you run this it will take some extra time to download the necessary dependencies into your local Nix store.
+3. Run `sbt`, `jekyll`, etc as required from the `nix-shell`.
+4. When you are finished you can `exit` the `nix-shell`.

shell.nix

@@ -0,0 +1,25 @@
+let
+
+  # use a pinned version of nixpkgs for reproducability
+  nixpkgs-version = "18.09";
+  pkgs = import (builtins.fetchTarball {
+    # Descriptive name to make the store path easier to identify
+    name = "nixpkgs-${nixpkgs-version}";
+    url = "https://github.com/nixos/nixpkgs/archive/${nixpkgs-version}.tar.gz";
+    # Hash obtained using `nix-prefetch-url --unpack <url>`
+    sha256 = "1ib96has10v5nr6bzf7v8kw7yzww8zanxgw2qi1ll1sbv6kj6zpd";
+  }) {};
+in
+  with pkgs;
+  stdenv.mkDerivation {
+    name = "cats-dev-env";
+
+    buildInputs = [
+      sbt
+      git # used by sbt-buildinfo
+      nodejs # used by scala.js
+      jekyll # used by sbt-microsites
+      gawk # used by scripts/parse-test-durations.awk
+      graphviz # used for ScalaDoc diagrams
+    ];
+  }