README improvements

Author: danvick

CHANGELOG.md

@@ -1,5 +1,5 @@
 ## [3.9.0] - 03-May-2020
-* New field type `FormBuilderImagePicker`
+* New field type `FormBuilderImagePicker` courtesy of [Gustavo Vítor](https://github.com/gustavovitor)
 * Switched rating package from [sy_flutter_widgets](https://pub.dev/packages/form_builder_map_field) to [sy_flutter_widgets](https://pub.dev/packages/rating_bar) with more configuration options
 * Added option to `showCheckmark` for FormBuilderFilterChip, along with other options. Closes #260
 * Added option to `allowEmpty` in `minLength` and `maxLength` validations. Closes #259