Update upgrade documentation

docs/en/user/upgrade-2.1.x-2.1.y.rst

@@ -0,0 +1,35 @@
+Upgrading from 2.1.x to 2.1.y
+=============================
+
+Upgrade on a dedicated web server
+---------------------------------
+
+The last release is published on https://www.wallabag.org/pages/download-wallabag.html. In order to upgrade your wallabag installation and get the last version, run the following commands in you wallabag folder (replace ``2.1.2`` by the last release number):
+
+::
+
+    rm -rf var/cache/*
+    git fetch origin
+    git fetch --tags
+    git checkout 2.1.2 --force
+    SYMFONY_ENV=prod composer install --no-dev -o --prefer-dist
+    php bin/console cache:clear --env=prod
+
+Upgrade on a shared hosting
+---------------------------
+
+Backup your ``app/config/parameters.yml`` file.
+
+Download the last release of wallabag:
+
+.. code-block:: bash
+
+    wget http://wllbg.org/latest-v2-package && tar xvf latest-v2-package
+
+(md5 hash of the 2.1.1 package: ``9584a3b60a2b2a4de87f536548caac93``)
+
+Extract the archive in your wallabag folder and replace ``app/config/parameters.yml`` with yours.
+
+If you use SQLite, you must also copy your ``data/`` folder inside the new installation.
+
+Empty ``var/cache`` folder.