redhat: add documentation about the os-build rebase process

We have been rebasing os-build lately on each new major Linux upstream
release. Add general instructions/guideline on how to do the rebase
process on top of last upstream tag/commit.

This also provides an explanation on why we are doing the rebase,
provided by Don Zickus. Added some of his suggestions as well on
the rebase process instructions.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>