add to upstream and add a short pulling section

Author: schacon

book/03-git-branching/1-git-branching.asc

@@ -743,6 +743,28 @@ Switched to a new branch 'sf'
 ----
 
 Now, your local branch sf will automatically push to and pull from origin/serverfix.
+If you already have a local branch and want to set it to a remote branch you just pulled down, or want to change the upstream branch you're tracking, you can use the `-u` or `--set-upstream-to` option to `git branch` to explicitly set it at any time.
+
+[source,shell]
+----
+$ git branch -u origin/serverfix
+Branch serverfix set up to track remote branch serverfix from origin.
+----
+
+[NOTE]
+.Upstream shorthand
+====
+When you have an tracking branch set up, you can reference it with the `@{upstream}` or `@{u}` shorthand. So if you're on the `master` branch and it's tracking `origin/master`, you can say something like `git merge @{u}` instead of `git merge origin/master` if you wish.
+====
+
+==== Pulling
+
+While the `git fetch` command will fetch down all the changes on the server that you don't have yet, it will not modify your working directory at all.
+It will simply get the data for you and let you merge it yourself.
+However, there is a command called `git pull` which is essentially a `git fetch` immediately followed by a `git merge` in most cases.
+If you have a tracking branch set up as demonstrated in the last section, either by explicitly setting it or by having it created for you by the `clone` or `checkout` commands, `git pull` will look up what server and branch your current branch is tracking, fetch from that server and then try to merge in that remote branch.
+
+There are other uses of the `pull` command that we'll address in [[_git_tools]], but generally it's better to simply use the `fetch` and `merge` commands explicitly as  the magic of `git pull` can often be confusing.
 
 ==== Deleting Remote Branches