Add CONTRIBUTING.md

This is just a really basic document from the info you gave me, but it's a
start.

dmjio#96 (comment)

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+To setup your development environment, you'll need the [Nix package manager](https://nixos.org/nix/).
+
+1. Clone the repo:
+   ```
+   git clone git@github.com:dmjio/stripe.git
+   cd stripe
+   ```
+2. Get into a shell for stripe-http-streams (stripe-tests is used in the test suite of stripe-http-streams).
+   ```
+   nix-shell --attr stripe-http-streams.env
+   cd stripe-http-streams
+   ```
+3. Ensure dependencies are all correct (this is contingent on your nixpkgs
+   version). Bump the upper bounds in the cabal files if need be.
+4. Configure and run tests using cabal.
+   ```
+   cabal configure --enable-tests
+   cabal test
+   ```