Add Callback docs

docs/Basic.md

@@ -25,3 +25,4 @@ From the code above, you can see that you can use `Tripay` facade to call the fu
 More docs:
 - [Payment](https://github.com/teikun-86/tripay-laravel/tree/main/docs/Payment.md)
 - [PPOB](https://github.com/teikun-86/tripay-laravel/tree/main/docs/PPOB.md)
+- [Handling Callback](https://github.com/teikun-86/tripay-laravel/tree/main/docs/Callback.md)

docs/Callback.md

@@ -0,0 +1,43 @@
+<h2>Callback</h2>
+
+There is 2 type of callbacks, PPOB Callback, and Payment Callback. But we only need 1 method to get the callback. That is `tripayCallback()` function that can be called globally, inside the method, we check the callback type, if the callback type is PPOB, then the instance returned is `PPOBCallback`, otherwise, it will return `Callback`.
+
+Let's start with creating the instance.
+
+```php
+<?php
+...
+$callback = tripayCallback();
+```
+
+Both `PPOBCallback` and `Callback` has `payloads()` method that will return the Callback's payload.
+For example:
+
+```php
+<?php
+...
+$payloads = $callback->payloads();
+```
+
+Callback example as JSON:
+
+```json
+{
+	"trxid": "158561****",
+	"api_trxid": "INV45769",
+	"via": "API",
+	"code": "S100",
+	"produk": "Telkomsel 100",
+	"harga": "97765",
+	"target": "08522083****",
+	"mtrpln": "-",
+	"note": "Trx S100 08522083**** SUKSES. SN: 845392759476503****",
+	"token": "845392759476503****",
+	"status": "1",
+	"saldo_before_trx": "100000",
+	"saldo_after_trx": "5894",
+	"created_at": "2019-11-06 12:07:48",
+	"updated_at": "2019-11-15 20:59:10",
+	"tagihan": null
+}
+```