add optional "defunct" property to guide

CONTRIBUTING.md

@@ -2,18 +2,23 @@
 
 1. Fork the [Banks DB](https://github.com/Ramoona/banks-db) repository and then clone your fork.
 2. Create new JSON file in `banks` folder and name it same as your bank title (lowercase).
-3. Fill your JSON with this guide (all properties are required):
-    * `name`: bank codename in lowercase (same as your file name)
-    * `prefixes`: array of bank prefixes (each prefix must contain 5 or 6 digits)
-    * `country`: two letters code of bank country ([ISO 3166-1 alpha-2](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2))
-    * `localTitle`: bank title in local language
-    * `engTitle`: bank title in English
-    * `url`: link to official bank website
-    * `color`: bank brand color from official logo or website
+3. Fill your JSON with this guide:
 
-    Here's an example:
+  * Required:
+      * `name`: bank codename in lowercase (same as your file name)
+      * `prefixes`: array of bank prefixes (each prefix must contain 5 or 6 digits)
+      * `country`: two letters code of bank country ([ISO 3166-1 alpha-2](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2))
+      * `localTitle`: bank title in local language
+      * `engTitle`: bank title in English
+      * `url`: link to official bank website
+      * `color`: bank brand color from official logo or website
+
+  * Optional  
+    * `defunct` : set `true` for banks with revoked license
 
-    ```js
+  Here's an example:
+
+  ```js
     {
       "name": "mybank",
       "prefixes": [