Add README, fix license to AGPLv3, untrack sync.sh

Add project README with features, installation instructions, and ELK
layout explanation. Fix license identifier from GPL-3.0 to AGPL-3.0 in
composer.json and module.php to match the actual LICENSE file. Remove
sync.sh from version control and add it to .gitignore.

module.php

@@ -3,7 +3,7 @@
 /**
  * Full Diagram module for webtrees.
  *
- * @license GPL-3.0-or-later
+ * @license AGPL-3.0-or-later
  */
 
 declare(strict_types=1);