Added ignore of W3TC cache folder (for users that might be using W3TC cache plugin)

Wordpress.gitignore

@@ -11,6 +11,7 @@ wp-content/advanced-cache.php
 wp-content/wp-cache-config.php
 wp-content/cache/*
 wp-content/cache/supercache/*
+wp-content/w3tc/*
 sitemap.xml
 sitemap.xml.gz
 readme.html