update read me (#384)

* update read me

close #381

* add config

* add stripe version for 2.x

* reword

README.md

@@ -18,6 +18,8 @@ The old `1.x.x` line of releases has been kept and is being published separately
 
 The actively developed line of releases is `2.x.x` and is contained within the `master` branch. New features are being added to this line of releases, so to develop this library further, create pull requests against the master branch.
 
+2.x.x Works with Stripe API version 2018-05-21
+
 # Documentation
 
 - [Latest HexDocs](https://hexdocs.pm/stripity_stripe/)
@@ -40,6 +42,15 @@ _Not necessary if using elixir >= 1.4_
 defp application do
   [applications: [:stripity_stripe]]
 end
+
+## Configuration
+
+To make API calls, it is necessary to configure your Stripe secret key.
+
+```ex
+use Mix.Config
+
+config :stripity_stripe, api_key: "YOUR SECRET KEY"
 ```
 
 # Documentation for 1.x.x