ISSUE-247. Updated README to document how payload is used. (#255)

README.md

@@ -88,13 +88,21 @@ public class SomeActivity extends Activity implements BillingProcessor.IBillingH
 ```
 
 * Call `purchase` method for a BillingProcessor instance to initiate purchase or `subscribe` to initiate a subscription:
+
+_Without a developer payload:_
 ```java
 bp.purchase(YOUR_ACTIVITY, "YOUR PRODUCT ID FROM GOOGLE PLAY CONSOLE HERE");
 bp.subscribe(YOUR_ACTIVITY, "YOUR SUBSCRIPTION ID FROM GOOGLE PLAY CONSOLE HERE");
 ```
+_With a developer payload:_
+```java
+bp.purchase(YOUR_ACTIVITY, "YOUR PRODUCT ID FROM GOOGLE PLAY CONSOLE HERE", "DEVELOPER PAYLOAD HERE");
+bp.subscribe(YOUR_ACTIVITY, "YOUR SUBSCRIPTION ID FROM GOOGLE PLAY CONSOLE HERE", "DEVELOPER PAYLOAD HERE");
+```
+_IMPORTANT: when you provide a payload, internally the library prepends a string to your payload. For subscriptions, it prepends `"subs:\<productId\>:"`, and for products, it prepends `"inapp:\<productId\>:\<UUID\>:"`. This is important to know if you do any validation on the payload returned from Google Play after a successful purchase._
 * **That's it! A super small and fast in-app library ever!**
 
-* **And dont forget**
+* **And don't forget**
  to release your BillingProcessor instance! 
 ```java
 	@Override