Add documentation for 'eventProp'

Author: jarvelov

docs/SUMMARY.md

@@ -14,6 +14,7 @@
         * [dynamicOptions](api/ui-schema/field/dynamic-options.md)
         * [errorHandler](api/ui-schema/field/error-handler.md)
         * [errorOptions](api/ui-schema/field/error-options.md)
+        * [eventProp](api/ui-schema/field/event-prop.md)
         * [fieldOptions](api/ui-schema/field/field-options.md)
           * [attrs](api/ui-schema/field/field-options/attrs.md)
           * [class](api/ui-schema/field/field-options/class.md)

docs/api/ui-schema/field.md

@@ -12,6 +12,7 @@ A `field` is an `object` and must have a [component](field/component.md) propert
 * [dynamicOptions](field/dynamic-options.md)
 * [errorHandler](field/error-handler.md)
 * [errorOptions](field/error-options.md)
+* [eventProp](field/event-prop.md)
 * [fieldOptions](field/field-options.md)
 * [valueProp](field/value-prop.md)
 

docs/api/ui-schema/field/event-prop.md

@@ -0,0 +1,35 @@
+# Event prop
+
+Sets which property should be emitted as the field's value when the component emits an event which is based on the [Event](https://developer.mozilla.org/en-US/docs/Web/API/Event) interface. The property specified by `eventProp` is read from the event's `target` property, i.e. `event.target[eventProp]`. Default value is `value`, i.e. `event.target['value']`.
+
+## Set globally for all fields
+
+```js
+data() {
+  return {
+    options: {
+      eventProp: 'target.href'
+    }
+  }
+}
+```
+
+## Set locally for one field
+
+```js
+data() {
+  return {
+    uiSchema: [{
+      component: 'input',
+      model: 'isChecked',
+      eventProp: 'checked',
+      valueProp: 'checked',
+      fieldOptions: {
+        type: "checkbox",
+        on: ['input']
+      }
+    }]
+
+  }
+}
+```