Added responsive image tag

Contributed-apps-and-website-code.md

@@ -80,4 +80,10 @@ Also on Github:
 - https://github.com/springload/wagtail-django-recaptcha
 - https://github.com/alexgleason/wagtailerrorpages
 - https://github.com/takeflight/wagtail-metadata
-- https://github.com/tomdyson/wagalytics (Google Analytics in Wagtail admin)
+- https://github.com/tomdyson/wagalytics (Google Analytics in Wagtail admin)
+
+## Snippets
+
+#### Media
+
+- https://gist.github.com/davecranwell/baf42b089d71b94fd229 (Responsive image tag)