docs: add simple 'add' case to clarify setup.

This patch adds a simple use case for adding a library to an existing
repository.

Signed-off-by: Dan Sabath <dsabath@whitepages.com>

git-subtree.txt

@@ -225,7 +225,31 @@ OPTIONS FOR split
 
 EXAMPLE 1
 ---------
-Let's use the repository for the git source code as an example. 
+Let's assume that you have a local repository that you would like
+to add an external vendor library to. In this case we will add the
+git-subtree repository as a subdirectory of your already existing
+git-extensions repository in ~/git-extensions/.
+
+First we need to fetch the remote objects
+	$ cd ~/git-extensions
+	$ git fetch git://github.com/apenwarr/git-subtree.git master
+
+'master' needs to be a valid remote ref and can be a different branch
+name
+
+Now we add the vendor library with
+	$ git subtree add --prefix=git-subtree --squash FETCH_HEAD
+
+You can omit the --squash flag, but doing so will increase the number
+of commits that are incldued in your local repository.
+
+We now have  ~/git-extensions/git-subtree directory with the git-subtree
+subdirectory containing code from the master branch of
+git://github.com/apenwarr/git-subtree.git
+
+EXAMPLE 2
+---------
+Let's use the repository for the git source code as an example.
 First, get your own copy of the git.git repository:
 
 	$ git clone git://git.kernel.org/pub/scm/git/git.git test-git
@@ -284,7 +308,7 @@ the standard gitweb:
 
 	git log gitweb-latest..$(git subtree split --prefix=gitweb)
 
-EXAMPLE 2
+EXAMPLE 3
 ---------
 Suppose you have a source directory with many files and
 subdirectories, and you want to extract the lib directory to its own