Subtree doc fix for committing code to subtree

git doesn't allow you to checkout a branch based upon a URL. You must
add it as a remote, fetch that remote, and then attempt to checkout a
branch based upon it.

Change-Id: I51ba9790dc40156c606221060ec68d6b0899a21c
Signed-off-by: Dave Cobbley <david.j.cobbley@linux.intel.com>
diff --git a/subtree.md b/subtree.md
index 96d645c..b366fa4 100644
--- a/subtree.md
+++ b/subtree.md
@@ -17,7 +17,9 @@
 # Make changes
 $ bitbake obmc-phosphor-image # Test out your changes
 $ git commit
-$ git checkout -b featureBranch ssh://openbmc.gerrit/openbmc/<repo_name>/master
+$ git remote add <repo_name> ssh://openbmc.gerrit/openbmc/<repo_name>/
+$ git fetch <repo_name>
+$ git checkout -b featureBranch <repo_name>/master
 $ git cherry-pick --strategy=subtree <SHA> # My commit from master
 $ git push ssh://openbmc.gerrit/openbmc/<repo_name> HEAD:refs/for/master