Add missing lines in update guide.

doc/update/6.8-to-6.9.md

@@ -52,6 +52,12 @@ sudo -u git -H bundle install --without development test postgres --deployment
 # PostgreSQL installations (note: the line below states '--without ... mysql')
 sudo -u git -H bundle install --without development test mysql --deployment
+
+# Run database migrations
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+
+# Clean up assets and cache
+sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS_ENV=production
 ```
 ### 5. Update config files