[UPDATE] Add documentation for rebasing when contributing to the docs

Author: AndoniLarz

contributing/documentation/overview.rst

@@ -136,9 +136,38 @@ even remove any content and do your best to comply with the
 
 **Step 6.** **Push** the changes to your forked repository:
 
+Rebase your branch
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Before submitting your PR, update your branch (needed if it takes you a
+while to finish your changes):
+
+.. code-block:: terminal
+
+    $ git checkout 6.x
+    $ git fetch upstream
+    $ git merge upstream/6.x
+    $ git checkout improve_install_article
+    $ git rebase 6.x
+
+.. tip::
+
+    Replace ``6.x`` with the branch you selected previously (e.g. ``5.4``).
+
+When doing the ``rebase`` command, you might have to fix merge conflicts.
+``git status`` will show you the *unmerged* files. Resolve all the conflicts,
+then continue the rebase:
+
+.. code-block:: terminal
+
+    $ git add ... # add resolved files
+    $ git rebase --continue
+
+Make sure that the docs still build correctly and push your branch remotely:
+
 .. code-block:: terminal
 
-    $ git push origin improve_install_article
+    $ git push --force origin improve_install_article
 
 The ``origin`` value is the name of the Git remote that corresponds to your
 forked repository and ``improve_install_article`` is the name of the branch you