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.