Add options section to README

README.md

@@ -27,7 +27,7 @@ yarn add objection-guid
 
 ```js
 // Import the plugin.
-const guid = require('objection-guid');
+const guid = require('objection-guid')();
 const Model = require('objection').Model;
 
 // Mixin the plugin.
@@ -45,6 +45,22 @@ console.log(item.id);
 // bbbe64b0-61a3-11e7-879a-67bb027591aa
 ```
 
+## Options
+
+
+**field:** Overrides the default field name of the generated guid (Default: `id`)
+
+**generateGuid:** Overrides the default function that generates the guid (Default: generates UUIDs v4 by default)
+
+These options can be provided when instantiating the plugin:
+
+```js
+const guid = require('objection-guid')(
+  field: 'foo',
+  generateGuid: () => new Promise(resolve => resolve('bar'))
+);
+```
+
 ## Tests
 
 Run the tests from the root directory: