If you don't have a Waffle badge in your readme, you can easily create a pull request with the badge code in the share popover by clicking the "Send pull request" button. It will create the pull request with our default badge, which includes all your columns and the number of cards.
You can find instructions to customize your badge with one or some columns and choose a different style in the "General" tab of your "Settings" screen.
If you already have Waffle's old badge, you won't find the above "Send pull request" button on your board, but your badge will still work. If you want to update, you can follow the instructions in the "General" tab of your "Settings" screen.