Document work-around for 'next' branch rebases.

Upstream in the copyleft.org tutorial repository, the next branch is
sometimes rebased against the master branch.  (For example, this occurs
when there have been quick fixes done on 'master' while new drafting
occurs on 'next'.)

This procedure, while convoluted, is the best way I've found to
compensate for this problem.  Hosting sites like Gitorious really aren't
designed for rebased branches.

CONTRIBUTING.md

@@ -125,17 +125,30 @@ request.
 0. First, ensure the Git repository points at the right branch and all old
    changes are committed.
 
-        $ git checkout official-next
-        $ git pull copyleft-tutorial-official
         $ git status
 
    The output of the last command should look like this:
 
-        # On branch official-next
+        # On branch SOME_BRANCH
         nothing to commit (working directory clean)
 
-   If you don't get that output, you probably have uncommitted changes, which
-   is beyond the scope of this tutorial.
+   If you don't get that output, you probably have uncommitted changes from
+   some other situation, which is beyond the scope of this document.
+
+1. Now, get the most recent version of the 'next' branch:
+
+        $ git checkout master
+        $ git branch -D official-next
+        $ git fetch copyleft-tutorial-official
+        $ git branch --track official-next copyleft-tutorial-official/next
+        $ git checkout official-next
+        $ git pull
+
+   (This step is more complicated than is typically found in a tutorial like
+   this.  Experienced Git users will note the above is the easiest way to
+   handle the fact that the 'next' branch is occasionally rebased against
+   master.  If 'next' branch has not been rebased since the last time the
+   operation was performed, the last two commands suffice for this step.)
 
 1. Next, create a new branch to hold your changes: