add section on syncing to the walkthrough

doc/walkthrough.mdwn

@@ -8,6 +8,7 @@ A walkthrough of the basic features of git-annex.
 	adding_files
 	renaming_files
 	getting_file_content
+	syncing
 	transferring_files:_When_things_go_wrong
 	removing_files
 	removing_files:_When_things_go_wrong

doc/walkthrough/getting_file_content.mdwn

@@ -10,13 +10,3 @@ USB drive.
 	# git annex get .
 	get my_cool_big_file (from laptop...) ok
 	get iso/debian.iso (from laptop...) ok
-
-Notice that you had to git fetch and merge from laptop first, this lets
-git-annex know what has changed in laptop, and so it knows about the files
-present there and can get them.
-
-The alternate approach is to set up a
-[[central bare repository|tips/centralized_git_repository_tutorial]], and
-always push changes to it after committing them, then in the above,
-you can just pull from the central repository to get synced up to
-all repositories.

doc/walkthrough/syncing.mdwn

@@ -0,0 +1,25 @@
+Notice that in the [[previous example|getting_file_content]], you had to
+git fetch and merge from laptop first. This lets git-annex know what has
+changed in laptop, and so it knows about the files present there and can
+get them.
+
+If you have a lot of repositories to keep in sync, manually fetching and 
+merging from them can become tedious. To automate it there is a handy
+sync command, which also even commits your changes for you.
+
+	# cd /media/usb/annex
+	# git annex sync
+	commit
+	nothing to commit (working directory clean)
+	ok
+	pull laptop
+	ok
+	push laptop
+	ok
+
+After you run sync, the repository will be updated with all changes made to
+its remotes, and any changes in the repository will be pushed out to its
+remotes, where a sync will get them. This is especially useful when using
+git in a distributed fashion, without a 
+[[central bare repository|tips/centralized_git_repository_tutorial]]. See
+[[sync]] for details.