add comment explaining built_tag use

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

conmon.spec

@@ -18,6 +18,8 @@
 %global shortcommit0 %(c=%{commit0}; echo ${c:0:7})
 %global git0 https://%{import_path}
 
+# Used for comparing with latest upstream tag
+# to decide whether to autobuild (non-rawhide only)
 %global built_tag v2.0.0
 
 Name: %{repo}