docs/manual/contribute.txt: add section for preparing patch series

In subsequent patches, we will add more explanation about how to
prepare patches, so it will be worthwhile to have a separate section
for the series preparation.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas: rewrap to our normal formatting practice.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/contribute.txt

@@ -182,10 +182,13 @@ _Please, do not attach patches to bugs, send them to the mailing list
 instead_.
 
 If you made some changes to Buildroot and you would like to contribute
-them to the Buildroot project, proceed as follows. Starting from the
-changes committed in your local git view, _rebase_ your development
-branch on top of the upstream tree before generating a patch set. To do
-so, run:
+them to the Buildroot project, proceed as follows.
+
+==== Preparing a patch series
+
+Starting from the changes committed in your local git view, _rebase_
+your development branch on top of the upstream tree before generating
+a patch set. To do so, run:
 
 ---------------------
 $ git fetch --all --tags