fix allow_resize and document it

README.md

@@ -37,6 +37,7 @@ let g:lua_tree_hide_dotfiles = 1 "0 by default, this option hides files and fold
 let g:lua_tree_git_hl = 1 "0 by default, will enable file highlight for git attributes (can be used without the icons).
 let g:lua_tree_root_folder_modifier = ':~' "This is the default. See :help filename-modifiers for more options
 let g:lua_tree_tab_open = 1 "0 by default, will open the tree when entering a new tab and the tree was previously open
+let g:lua_tree_allow_resize = 1 "0 by default, will not resize the tree when opening a file
 let g:lua_tree_show_icons = {
     \ 'git': 1,
     \ 'folders': 0,