Update readme with video walkthrough.

git-svn-id: https://plugins.svn.wordpress.org/menu-social-icons/trunk@765364 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update YouTube URL for oEmbed

git-svn-id: https://plugins.svn.wordpress.org/menu-social-icons/trunk@765365 b8457f37-d9ea-0310-8a92-e5e31aec5664