document options

Author: nathanstitt

README.md

@@ -105,6 +105,7 @@ Props:
   * maxlength,
   * minlength
   * select
+  * options - an object containing any other valid Braintree options such as maskInput
   * prefill
 
 See the [Braintree api docs](https://braintree.github.io/braintree-web/3.33.0/module-braintree-web_hosted-fields.html#%7Efield) for more details