Update intstructions for existing nvim setups

If you already had a nvim setup, then this makes it so that the kickstarter can work without a problem. Having that file exist and loaded with plugins gives the script many issues.

Also better to have it be a shell command rather than have the script delete a bunch of files on your computer.

README.md

@@ -18,6 +18,8 @@ This repo is meant to be used as a starting point for a user's own configuration
 * Start Neovim (`nvim`) and run `:PackerInstall` - ignore any error message about missing plugins, `:PackerInstall` will fix that shortly
 * Restart Neovim
 
+N.B. if you already had a prior nvim setup with plugins you will need to remove them first with `rm -rf ~/.local/share/nvim/`
+
 ### Configuration
 
 You could directly modify the `init.lua` file with your personal customizations. This option is the most straightforward, but if you update your config from this repo, you may need to reapply your changes.