Add friendly comment to the config file

diff --git a/config.in b/config.in
index e75ef07..2278fcf 100644
--- a/config.in
+++ b/config.in
@@ -1,3 +1,4 @@
+# See soju(1) for supported directives
 db sqlite3 /var/lib/soju/main.db
 message-store fs /var/lib/soju/logs/
 listen ircs://