Restart Startify after plugin installation on startup (tip)

This is not a bug, just a suggestion for user configuration, so feel free to close this one immediately.

If you are using Vim-Plug, you probably have something like this in your vimrc to install plugins on startup. However, this version also closes the buffer again after that and shows Startify:

```
" Run PlugInstall if there are missing plugins
augroup plugins_check
autocmd VimEnter * if len(filter(values(g:plugs), '!isdirectory(v:val.dir)'))
\| PlugInstall --sync | source $MYVIMRC | execute 'close' | execute 'Startify'
\| endif
augroup END
```

The issue provides a Vim-Plug autocmd for use in a vimrc, but names no repository file or test. Start by checking whether user configuration tips are documented in the project; done means deciding where this tip belongs and recording the provided startup configuration if appropriate.

Written by the indexing model from the issue text.