Update instructions for configuring Neoformat (#11946)

Co-authored-by: fisker Cheung <lionkay@gmail.com>

docs/vim.md

@@ -17,6 +17,12 @@ The best way to install Neoformat is with your favorite plugin manager for Vim,
 Plug 'sbdchd/neoformat'
 ```
 
+In order for Neoformat to use a project-local version of Prettier (i.e. to use `node_modules/.bin/prettier` instead of looking for `prettier` on `$PATH`), you must set the `neoformat_try_node_exe` option:
+
+```vim
+let g:neoformat_try_node_exe = 1
+```
+
 Run `:Neoformat` or `:Neoformat prettier` in a supported file to run Prettier.
 
 To have Neoformat run Prettier on save: