add intro comment in config

_config.yml

@@ -1,3 +1,8 @@
+# Welcome to Beautiful Jekyll!
+# This config file is meant for settings that affect your entire website. When you first
+# set up your website you should go through all these settings and edit them, but after
+# the initial set up you won't need to come back to this file often.
+
 # --- Required options --- #
 
 # Name of website