Add archive_command example

Author: petere

src/backend/utils/misc/postgresql.conf.sample

@@ -186,6 +186,9 @@
 #archive_mode = off		# allows archiving to be done
 				# (change requires restart)
 #archive_command = ''		# command to use to archive a logfile segment
+				# placeholders: %p = path of file to archive
+				#               %f = file name only
+				# e.g. 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'
 #archive_timeout = 0		# force a logfile segment switch after this
 				# number of seconds; 0 disables