give a hint about git rebase

It seems common knowledge to use git rebase in interactive mode
to fixup issues while respinning patch series, but I found it hard
to find any hint about it. Add a note for git beginners like me.

[Thomas: take into account Arnout's comment.]

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>