Add sf Live logo

Might be a bit small, I do not know.

hello.css

@@ -11,6 +11,9 @@ code {
 
 .reveal {
     color: white;
+    background-image: url(./sflive_paris2018_big_transp.png);
+    background-repeat: no-repeat;
+    background-position: top right;
 }
 
 @font-face {