Add discourse example in setup.cfg

As per other data sources, it may be useful to newcomers to have a
commented out configuration for the discourse backend.

Signed-off-by: Mike Nolan <me@michael-nolan.com>

default-grimoirelab-settings/setup.cfg

@@ -57,6 +57,11 @@ panels = true
 #raw_index = confluence_demo_raw
 #enriched_index = confluence_demo_enriched
 
+#[discourse]
+#raw_index = discourse_demo_raw
+#enriched_index = discourse_demo_enriched
+#no-archive = true
+
 [git]
 raw_index = git_demo_raw
 enriched_index = git_demo_enriched