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>

docs/manual/contribute.txt

@@ -222,6 +222,9 @@ should include a changelog of the modifications between each
 submission. Note that when your patch series is introduced by a cover
 letter, an overall changelog may be added to the cover letter in
 addition to the changelog in the individual commits.
+The best thing to rework a patch series is by interactive rebasing:
++git rebase -i origin/master+. Consult the git manual for more
+information.
 
 When added to the individual commits, this changelog is added when
 editing the commit message.  Below the +Signed-off-by+ section, add