Add new upgrade instructions

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

upgrading/4.9.0.txt → upgrading/5.x.x.txt

+Starting from release 5.0.0, Alembic is used for managing database migrations.
+
+Run `alembic upgrade head` from the aurweb root directory to upgrade your
+database after upgrading the source code to a new release.
+
+When upgrading from 4.8.0, you also need to execute the following manual SQL
+statements before doing so.
+
 1. Add new columns to store the timestamp and UID when closing requests:
 
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