Update source file contribute.html

lang/en/texts/contribute.html

@@ -40,7 +40,7 @@ <h4 class="subheader">Be part of our collaborative, free and open source databas
 <div class="row text-center">
 	<div class="medium-3 columns">
 		<h3>Add products</h3>
-		<p>Use our <a href="https://play.google.com/store/apps/details?id=org.openfoodfacts.scanner&hl=en">Android</a>, <a href="https://apps.apple.com/app/open-food-facts/id588797948">iPhone</a> or <a href="https://apps.microsoft.com/store/detail/open-food-facts-scan-to-get-nutriscore-ecoscore-and-more/XP8LT18SRPKLRG">Windows</a> app to easily scan the barcode of products from your home or local stores and upload pictures of their label.</p>
+		<p>Use our <a href="https://play.google.com/store/apps/details?id=org.openfoodfacts.scanner&hl=en&utm_source=off&utm_campaign=contribute-promo&utm_medium=website">Android</a>, <a href="https://apps.apple.com/app/open-food-facts/id588797948?utm_source=off&utm_campaign=contribute-promo&utm_medium=website">iPhone</a> or <a href="https://apps.microsoft.com/store/detail/open-food-facts-scan-to-get-nutriscore-ecoscore-and-more/XP8LT18SRPKLRG?utm_source=off&utm_campaign=contribute-promo&utm_medium=website">Windows</a> app to easily scan the barcode of products from your home or local stores and upload pictures of their label.</p>
 		<p>No smartphone? No problem: you can also use your camera to add products directly on the web site.</p>
 	</div>