Add url for help link on mobile

Author: Nick Litwin

app/submissions/submit-file/submit-file.jade

@@ -4,7 +4,7 @@
   .mobile-redirect__body
     p Our team is working hard on new features, and file upload currently only works on the web. Please open this page on your desktop if you want to create a submission.
 
-    a(ng-href="TODO: Need Link") Learn more about formatting your submission file
+    a(ng-href="http://help.{{DOMAIN}}/design/submitting-to-a-design-challenge/formatting-your-submission-for-design-challenges/") Learn more about formatting your submission file
 
     a.tc-btn.tc-btn-s(ng-href="https://www.{{DOMAIN}}/challenge-details/{{submissions.challengeId}}/?type={{submissions.track}}") Back to Challenge Details