docs: update readme, mention iframely support

[we're whitelisted on iframely!](https://iframely.com/embed/https%3A%2F%2Ftesting-playground.com%2F%3Fmarkup%3DDwIwrgLhD2B2B8AoANtA5gS1o4B6cUc8QA%26query%3DM4YwTgphB2B0DmEAuAhAngJQPYBsIAoByAIwFckktpCBKIA%26panes%3Dmarkup%2Cquery)

![image](https://user-images.githubusercontent.com/1196524/83190483-c4c67a80-a132-11ea-911b-e8277cb38fed.png)
