add the doc

Author: kyazdani42

doc/nvim-tree-lua.txt

@@ -446,8 +446,11 @@ Here is a list of the options available in the setup call:
         default: `true`
 
     - |renderer.icons.git_placement|: place where the git icons will be rendered.
-	After or before the `filename` (after the file/folders icons).
-	type: `after` or `before`
+      After or before the `filename` (after the file/folders icons).
+      When placed in the signcolumn, the value of signcolumn should be `yes`
+      for the nvim-tree window.
+      Note that the diagnostic signs will take precedence over the git signs.
+	type: `"after"`, `"before"` or `"signcolumn"`
 	default: `before`
 
 *nvim-tree.filters*