Update readme

Author: VincentBean

README.md

@@ -29,6 +29,7 @@ A Magento 2 API Object Oriented wrapper for a Laravel application.
   - [Products](#products)
   - [Schema](#schema)
   - [Source Items](#source-items)
+  - [Custom modules](#custom modules)
 
 
 ## Installation
@@ -223,6 +224,25 @@ Get info about a product by the product SKU:
 $magento->api('products')->show($sku);
 ```
 
+### Custom modules
+Magento modules can have their own API endpoints.
+For example:
+```xml
+<route method="POST" url="/V1/custom/save">
+    ...
+</route>
+<route method="GET" url="/V1/custom/get/:id">
+    ...
+</route>
+```
+To use these you can directly use get/post methods:
+```php
+$magento->api('custom')->post('save', [...]);
+```
+```php
+$magento->api('custom')->get('get/1');
+```
+
 <a id="schema"></a>
 ### Schema
 
@@ -265,4 +285,4 @@ If you discover any security related issues, please email webmaster@grayloon.com
 
 ## License
 
-The MIT License (MIT). Please see [License File](LICENSE.md) for more information.
+The MIT License (MIT). Please see [License File](LICENSE.md) for more information.