add img prop (#291)

Author: dks333

content/components/cards.mdx

@@ -67,3 +67,7 @@ Add an `img` property to a card to display an image on the top of the card.
 <ResponseField name="horizontal" type="boolean">
   Makes the card more compact and horizontal
 </ResponseField>
+
+<ResponseField name="img" type="string">
+  The url or local path to an image to display on the top of the card
+</ResponseField>