Add contributor doc

[aturkewi]

Add docs around how to contribute and how to get things up and running.

• [ ] Setting up stripe-mock
• [ ] Note any version(s) of elixir needed?

cc @snewcomer - conversation started over in #434

[aturkewi]

I also notice that the following tests fail when I run mix test on local. Perhaps there are more setup steps involved?

1) test retrieve/2 retrieves an file (Stripe.FileUploadTest)
2) test list/2 lists all files (Stripe.FileUploadTest)
3) test attach/2 attaches a source to a customer (Stripe.SourceTest)
4) test detach/2 detaches a source from a customer (Stripe.SourceTest)