Prepare the plugin for localization

Hi @jeffparker, thank you for the plugin!

We translated your plugin to Russian https://translate.ww.wp.xz.cn/locale/ru/default/wp-plugins/yet-another-related-posts-plugin.

But there is error: Your plugin slug is yet-another-related-posts-plugin, but there seems no Text Domain: defined in yarpp.php! To correctly internationlize your plugin you will need it.

Could you please fix this to prepare the plugin for localization? Thanks!