docs: Add showDropdown prop description to readme (#152)

Closes #144

Author: mrchief

README.md

@@ -50,6 +50,7 @@ A lightweight and fast control to render a select component that can display hie
   - [keepTreeOnSearch](#keeptreeonsearch)
   - [simpleSelect](#simpleselect)
   - [showPartiallySelected](#showpartiallyselected)
+  - [showDropdown](#showDropdown)
 - [Styling and Customization](#styling-and-customization)
   - [Using default styles](#default-styles)
   - [Customizing with Bootstrap, Material Design styles](#customizing-styles)
@@ -287,6 +288,12 @@ Type: `bool` (default: `false`)
 
 If set to true, shows checkboxes in a partial state when one, but not all of their children are selected. Allows styling of partially selected nodes as well, by using [:indeterminate](https://developer.mozilla.org/en-US/docs/Web/CSS/:indeterminate) pseudo class. Simply add desired styles to `.node.partial .checkbox-item:indeterminate { ... }` in your CSS.
 
+### showDropdown
+
+Type: `bool` (default: `false`)
+
+If set to true, shows the dropdown when rendered. This can be used to render the component with the dropdown open as its initial state.
+
 ## Styling and Customization
 
 ### Default styles