Hide “This site is open source. Improve this page.” footer

This hides the message shown at the bottom of pages served from
https://google.github.io/styleguide.

assets/css/style.scss

@@ -0,0 +1,9 @@
+---
+---
+
+@import "{{ site.theme }}";
+
+// Hide “This site is open source. Improve this page.”
+.footer {
+  display: none;
+}