Added documentation and missing translations

docs/en/developer/paywall.rst

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+Articles behind a paywall
+=========================
+
+wallabag can fetch articles from websites which use a paywall system.
+
+Enable paywall authentication
+-----------------------------
+
+In internal settings, in the **Article** section, enable authentication for websites with paywall (with the value 1).
+
+Configure credentials in wallabag
+---------------------------------
+
+Edit your ``app/config/parameters.yml`` file to edit credentials for each website with paywall. Here is an example for some french websites:
+
+.. code:: yaml
+
+    sites_credentials:
+        mediapart.fr: {username: "myMediapartLogin", password: "mypassword"}
+        arretsurimages.net: {username: "myASILogin", password: "mypassword"}
+
+.. note::
+
+    These credentials will be shared between each user of your wallabag instance.
+
+Parsing configuration files
+---------------------------
+
+.. note::
+
+    Read `this part of the documentation <http://doc.wallabag.org/en/master/user/errors_during_fetching.html>`_ to understand the configuration files.
+
+Each parsing configuration file needs to be improved by adding ``requires_login``, ``login_uri``,
+``login_username_field``, ``login_password_field`` and ``not_logged_in_xpath``.
+
+Be careful, the login form must be in the page content when wallabag loads it. It's impossible for wallabag to be authenticated
+on a website where the login form is loaded after the page (by ajax for example).
+
+``login_uri`` is the action URL of the form (``action`` attribute in the form).
+``login_username_field`` is the ``name`` attribute of the login field.
+``login_password_field`` is the ``name`` attribute of the password field.
+
+For example:
+
+.. code::
+
+    title://div[@id="titrage-contenu"]/h1[@class="title"]
+    body: //div[@class="contenu-html"]/div[@class="page-pane"]
+
+    requires_login: yes
+
+    login_uri: http://www.arretsurimages.net/forum/login.php
+    login_username_field: username
+    login_password_field: password
+
+    not_logged_in_xpath: //body[@class="not-logged-in"]