Add png logo favicon for site

Change-Id: I3e27ec18e8d07e9a42c5f826d77ae7b942f4e896
diff --git a/conf.py b/conf.py
index d8abd1b..104d20b 100644
--- a/conf.py
+++ b/conf.py
@@ -134,6 +134,10 @@
 #
 # html_sidebars = {}
 
+# favicon
+
+html_favicon = 'images/trellis-favicon-128.png'
+
 
 # -- Options for HTMLHelp output ---------------------------------------------