Adding the 'clearable' prop to the docs (#649)

What:

* Adding clearable to the gitbook Props.md

Why:

* Its a prop on the select component but it wasn't documented correctly

docs/gitbook/Api/Props.md

@@ -90,6 +90,15 @@ clearSearchOnSelect: {
 	default: true
 },
 
+/**
+ * Enables/disables allowing the user to clear the selected property.
+ * @type {Boolean}
+ */
+clearable: {
+  type: Boolean,
+  default: true
+},
+
 /**
  * Close a dropdown when an option is chosen. Set to false to keep the dropdown
  * open (useful when combined with multi-select, for example)