Adding github.io to the fronting domains (#747)

* Update config.fronting-groups.example.json

Added more Fastly fronting groups

* Update config.fronting-groups.example.json

Added `github.io` which is one of the most important domains on the github website

Author: Shjpr9

config.fronting-groups.example.json

@@ -44,6 +44,7 @@
 
         "githubassets.com",
         "githubusercontent.com",
+        "github.io",
 
         "pypi.org",