Add example of is_callable functionality.

See #205

example.php

@@ -78,6 +78,18 @@ function my_theme_register_required_plugins() {
 			'slug'      => 'buddypress',
 			'required'  => false,
 		),
+
+		// This is an example of the use of 'is_callable' functionality. A user could - for instance - 
+		// have WPSEO installed *or* WPSEO Premium. The slug would in that last case be different, i.e.
+		// 'wordpress-seo-premium'.
+		// By setting 'is_callable' to either a function from that plugin or a class method
+		// `array( 'class', 'method' )` similar to how you hook in to actions and filters, TGMPA can still
+		// recognize the plugin as being installed.
+		array(
+			'name'        => 'WordPress SEO by Yoast',
+			'slug'        => 'wordpress-seo',
+			'is_callable' => 'wpseo_init',
+		),
 
 	);