added missing package doc files

backup/doc.go

@@ -0,0 +1,2 @@
+// Package backup implements backup engine for files and directories.
+package backup

vault/doc.go

@@ -1 +1,2 @@
+// Package vault implements vault for securely storing object IDs and other sensitive data protected with a password or a key.
 package vault

vfs/doc.go

@@ -0,0 +1,2 @@
+// Package vfs implements FUSE filesystem interface based on files and directories stored in a repository.
+package vfs