Sentinel: Add "dir /tmp" directive in example sentinel.conf.

sentinel.conf

@@ -4,6 +4,13 @@
 # The port that this sentinel instance will run on
 port 26379
 
+# dir <working-directory>
+# Every long running process should have a well-defined working directory.
+# For Redis Sentinel to chdir to /tmp at startup is the simplest thing
+# for the process to don't interferer with administrative tasks such as
+# unmounting filesystems.
+dir /tmp
+
 # sentinel monitor <master-name> <ip> <redis-port> <quorum>
 #
 # Tells Sentinel to monitor this master, and to consider it in O_DOWN