docs: upgrade: add section on updating git hooks

Commit 39bdf0ab2862 has changed the Git hooks. When upgrading Kallithea to
this commit, but not explicitly reinstalling the Git hooks, pushing to Git
repos will fail.

Add a section to the documentation to explain that a reinstall of the hooks
may be needed, and how to do it.

docs/upgrade.rst

@@ -173,14 +173,32 b' need to append the following lines to it'
 Your config file should now work with Alembic.
 
 
-7. Rebuild the Whoosh full-text index
+7. Update Git repository hooks
+------------------------------
+
+It is possible that an upgrade involves changes to the Git hooks installed by
+Kallithea. As these hooks are created inside the repositories on the server
+filesystem, they are not updated automatically when upgrading Kallithea itself.
+
+To update the hooks of your Git repositories:
+
+* Go to *Admin > Settings > Remap and Rescan*
+* Select the checkbox *Install Git hooks*
+* Click the button *Rescan repositories*
+
+.. note::
+    Kallithea does not use hooks on Mercurial repositories. This step is thus
+    not necessary if you only have Mercurial repositories.
+
+
+8. Rebuild the Whoosh full-text index
 -------------------------------------
 
 It is recommended that you rebuild the Whoosh index after upgrading since
 new Whoosh versions can introduce incompatible index changes.
 
 
-8. Start the Kallithea web application
+9. Start the Kallithea web application
 --------------------------------------
 
 This step once again depends entirely on the web server software used to