doc: documented nvimtree transparent_panel opt usage

README.md

@@ -115,6 +115,7 @@ integrations = {
 	nvimtree = {
 		enabled = true,
 		show_root = false,
+		transparent_panel = false,
 	},
 	which_key = false,
 	indent_blankline = {
@@ -243,6 +244,7 @@ integration = {
   nvimtree = {
     enabled = true,
     show_root = true, -- makes the root folder not transparent
+	transparent_panel = false, -- make the panel transparent
   }
 }
 ```