Facebook Instant Articles custom transform rules

gistfile1.txt

{
    "rules" :
        [
            {
                "class": "HeaderSubTitleRule",
                "selector" : "header .subheadline"
            },
            {
            "class": "SocialEmbedRule",
            "selector" : ".entry-media figure iframe",
            "properties" : {
                "socialembed.url" : {
                    "type" : "string",
                    "selector" : "iframe",
                    "attribute": "src"
                }
             }
           }
        ]
}