Add README to payment protocol reference implementation.

examples/PayPro/README.md

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+# Running the Payment Protocol Demo
+This is an example of Bitcore's Payment Protocol implementation, including a
+mocked server (`server.js`) and client (`customer.js`).
+
+1. Start the server: `node server.js`
+2. Start the customer: `node customer.js`
+
+At this point, you should see an acknowledgement from your local server:
+
+```
+Customer: Our payment was acknowledged!
+Customer: Message from Merchant: Thank you for your payment!
+Customer: Payment sent successfully.
+```