Add showLoading() to TransactionCallback docs (#155)

* Add `showLoading()`  to `TransactionCallback` docs

* Update README.md

Co-authored-by: nathan-paystack <[email protected]>

---------

Co-authored-by: nathan-paystack <[email protected]>

Author: michael-paystack

README.md

@@ -214,6 +214,12 @@ public class MainActivity extends AppCompatActivity {
                // Save reference so you may send to server. If
                // error occurs with OTP, you should still verify on server.
            }
+           
+           @Override
+           public void showLoading(Boolean isProcessing) {
+               // This is called whenever the SDK makes network requests.
+               // Use this to display loading indicators in your application UI
+           }
 
            @Override
            public void onError(Throwable error, Transaction transaction) {