add Gitignore (#251)

doc/nvim-tree-lua.txt

@@ -68,6 +68,17 @@ useful to hide large data/cache folders.
 >
     example: let g:nvim_tree_ignore = [ '.git', 'node_modules' ]
 
+|g:nvim_tree_gitignore|				*g:nvim_tree_gitignore*
+
+Determines whether to include in g:nvim_tree_ignore
+files ignored by git.
+
+Must be:
+    0: not ignored
+    1: ignored files from .gitignore
+    2: ignored files from .gitignore and git exclude
+
+>
 |g:nvim_tree_show_icons|			        *g:nvim_tree_show_icons*
 
 Dictionary, if your terminal or font doesn't support certain unicode