add usage instructions

git-svn-id: https://weed-fs.googlecode.com/svn/trunk@40 282b0af5-e82d-9cf1-ede4-77906d7719d0

weed-fs/src/cmd/weedvolume/fix_volume_index/fix_volume_index.go

@@ -11,7 +11,7 @@ import (
 
 var (
 	dir      = flag.String("dir", "/tmp", "data directory to store files")
-	volumeId = flag.Int("volumeId", -1, "volume id")
+	volumeId = flag.Int("volumeId", -1, "a non-negative volume id. The volume should already exist in the dir. The volume index file should not exist.")
 	IsDebug  = flag.Bool("debug", false, "enable debug mode")
 
 	store *storage.Store