New doc: update installation

Explain how to update wallabag. Not tested yet, so not sure that a sudo chown -R www-data:www-data /var/www/html/wallabag/ is not necessary afterwhile.

Based on: http://support.wallabag.org/kb/faq/how-can-i-update-wallabag and http://support.wallabag.org/discussions/problems/30-update-wallabag-no-doc-available