Redis.conf example: make clear user must pass its path as argument.

redis.conf

@@ -1,4 +1,9 @@
-# Redis configuration file example
+# Redis configuration file example.
+#
+# Note that in order to read the configuration file, Redis must be
+# started with the file path as first argument:
+#
+# ./redis-server /path/to/redis.conf
 
 # Note on units: when memory size is needed, it is possible to specify
 # it in the usual form of 1k 5GB 4M and so forth: