Add friendly comment to the config file

diff --git a/config.in b/config.in
index a0b510e..056a466 100644
--- a/config.in
+++ b/config.in
@@ -1,2 +1,4 @@
+# See soju(1) for supported directives
+
 db sqlite3 /var/lib/soju/main.db
 log fs /var/lib/soju/logs/