readme update

README.md

@@ -183,6 +183,9 @@ You can customize the layout of the DropdownSearch and its items. [EXAMPLE](http
 |`popupItemDisabled`|defines if an item of the popup is enabled or not, if the item is disabled, it cannot be clicked|
 |`popupBarrierColor`|set a custom color for the popup barrier|
 |`searchBoxController`|search box controller|
+|`clearButtonBuilder`|custom clear button builder|
+|`dropdownButtonBuilder`|custom dropdown button builder|
+|`onBeforeChange`|callback executed before applying value change|
 |`searchDelay`|delay before searching|
 
 # Attention

lib/dropdown_search.dart

@@ -156,11 +156,12 @@ class DropdownSearch<T> extends StatefulWidget {
   ///called when popup is dismissed
   final VoidCallback onPopupDismissed;
 
+  /// callback executed before applying value change
   ///delay before searching, change it to Duration(milliseconds: 0)
   ///if you do not use online search
   final Duration searchDelay;
 
-  ///
+  /// callback executed before applying value change
   final BeforeChange<T> onBeforeChange;
 
   DropdownSearch({