Add docs and recipe for git submodules

[jason-murray]

Using git submodules for plugins and themes not on the wordpress.org repo is a great workflow for us and solves the issue of things not being on the .org repo quite nicely.

However there aren't any docs for this just now really, barring something in the TODO.md.

Also the deployer recipe can probably be adjusted a bit as the submodules are not in the .gitignore so are not symlinked as shared:

[jason-murray]

Hmm, learned the hard way here that git submodule (and git in general) won't follow symlinks.

Came up with this in regolith-recipe.php to get the list of submodules, but after they are symlinked deployer errors out during install-dependencies.sh

	$submodules = explode(PHP_EOL, `git submodule`);
	$submodule_dependencies = array ();

	foreach ($submodules as $str) {
		if ( preg_match( '#(web\/content\/.*) #', $str, $m ) ) {
			$submodule_dependencies[] = $m[1];
		}
	}
...
	$shared_directories = array_merge( $other_shared, $plugin_dependencies, $theme_dependencies, $submodule_dependencies );

[jason-murray]

I think we could add the submodules to the copy_dirs option in Deployer, to copy them between releases and save them being redownloaded every deploy.

[iandunn]

Hmm, I think when I got it working on one of my sites, I just let it download a fresh copy with each deploy. That's definitely slower, but the simplicity might make it worth the tradeoff.