Add image source attribution

Author: michielp1807

exercises/5-rust-for-web/3-rust-in-the-browser/1-lettuce-crop-wasm/assets/crop.txt

@@ -0,0 +1,4 @@
+The lettuce crop shown on our Lettuce Crop demo site is based on a photograph by Richard Revel, licensed CC0, available at
+https://www.publicdomainpictures.net/en/view-image.php?image=207177&picture=green-salad
+
+See more of his work here: https://www.publicdomainpictures.net/en/browse-author.php?page=1350&a=111549