Allow user-defined RO indicator.

(inspired by overwriting g:NERDTreeDirArrowCollapsible/Expandable)

Replace instances of the RO string with a variable that the user can
override. Useful for custom unicode glyphs, i.e. Font Awesome.

* Initialize variable g:NERDTreeGlyphReadOnly = "RO".
   -> plugin/NERD_tree.vim
* Replace instances of 'RO' with g:NERDTreeGlyphReadOnly

lib/nerdtree/path.vim

@@ -61,7 +61,7 @@ function! s:Path.cacheDisplayString() abort
     endif
 
     if self.isReadOnly
-        let self.cachedDisplayString .=  ' [RO]'
+        let self.cachedDisplayString .=  ' ['.g:NERDTreeGlyphReadOnly.']'
     endif
 endfunction