Update samplestore app README (#1473)

samplestore/README.md

@@ -1 +1,45 @@
-App icon from [Twemoji](https://github.com/twitter/twemoji)
+# Stripe Emoji Apparel App
+
+**Contents**
+1. [Setup](#setup)
+2. [Licenses](#licenses)
+
+## Setup
+
+### Install
+1. Clone the `stripe-android` repository.
+2. Open the project in Android Studio.
+3. After [deploying the example backend to Heroku](#deploy-the-example-backend-to-heroku) and [configuring the app](#configure-the-app), build and run the project.
+
+<img width="215" height="108" src="https://raw.githubusercontent.com/stripe/stripe-android/master/samplestore/assets/run.png" />
+
+### Deploy the example backend to Heroku
+1. [Create a Heroku account](https://signup.heroku.com/) if you don't have one.
+2. Navigate to the [example mobile backend repo](https://github.com/stripe/example-ios-backend)
+   and click "Deploy to Heroku".
+3. Set an _App Name_ of your choice (e.g. Stripe Example Mobile Backend).
+4. Under _Config Vars_, set your [Stripe testmode secret key](https://dashboard.stripe.com/test/apikeys)
+   for the `STRIPE_TEST_SECRET_KEY` field.
+5. Click "Deploy for Free".
+
+<img width="700" height="793" src="https://raw.githubusercontent.com/stripe/stripe-android/master/samplestore/assets/heroku.png" />
+
+### Configure the samplestore app
+1. Set [Settings.PUBLISHABLE_KEY](example/src/main/java/com/stripe/example/Settings.kt)
+   to your [test publishable key](https://dashboard.stripe.com/test/apikeys). 
+
+   For example,
+   ```
+   const val PUBLISHABLE_KEY = "pk_test_12345"
+   ```
+
+2. Set [Settings.BASE_URL](samplestore/src/main/java/com/stripe/samplestore/Settings.kt)
+   to the URL of the [example backend deployed to Heroku](#deploy-example-backend-to-heroku).
+
+   For example,
+   ```
+   const val BASE_URL = "https://my-example-app.herokuapp.com"
+   ```
+
+## Licenses
+- App icon from [Twemoji](https://github.com/twitter/twemoji)