Add support for RetainerInvoices module

src/Models/RetainerInvoice.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Webleit\ZohoBooksApi\Models;
+
+/**
+ * Class RetainerInvoice
+ * @package Webleit\ZohoBooksApi\Models
+ */
+class RetainerInvoice extends Model
+{
+
+}

src/Modules/RetainerInvoices.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Webleit\ZohoBooksApi\Modules;
+
+/**
+ * Class RetainerInvoices
+ * @package Webleit\ZohoBooksApi\Modules
+ */
+class RetainerInvoices extends Documents
+{
+
+}

src/ZohoBooks.php

@@ -14,6 +14,7 @@
  * @property-read Modules\SalesOrders $salesorders
  * @property-read Modules\Invoices $invoices
  * @property-read Modules\RecurringInvoices $recurringinvoices
+ * @property-read Modules\RetainerInvoices $retainerinvoices
  * @property-read Modules\CreditNotes $creditnotes
  * @property-read Modules\CustomerPayments $customerpayments
  * @property-read Modules\Expenses $expenses
@@ -55,6 +56,7 @@ class ZohoBooks implements Contracts\ProvidesModules
         'salesorders'            => Modules\SalesOrders::class,
         'invoices'               => Modules\Invoices::class,
         'recurringinvoices'      => Modules\RecurringInvoices::class,
+        'retainerinvoices'       => Modules\RetainerInvoices::class,
         'creditnotes'            => Modules\CreditNotes::class,
         'customerpayments'       => Modules\CustomerPayments::class,
         'expenses'               => Modules\Expenses::class,