Create 02_bis_Mettre_a_jour_l_installation.md

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.
Create 02_bis_Mettre_a_jour_installation.md

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.
Delete 02_bis_Mettre_a_jour_installation.md
Rename en/02_User_documentation/02_Download_and_install.md to en/02_Administrator_Documentation/01_Download_and_install.md

With new file tree proposal
Merge pull request #21 from etiess/master

New doc: update installation