Added documentation about errors during fetching

docs/en/user/errors_during_fetching.rst

@@ -0,0 +1,27 @@
+Errors during fetching articles
+===============================
+
+Why does the fetch of an article fail?
+--------------------------------------
+
+There may be several reasons:
+
+- network problem
+- wallabag can't fetch content due to the website structure
+
+How can I help to fix that?
+---------------------------
+
+- `by sending us an email with the article's URL <mailto:hello@wallabag.org>`_
+- by trying to fix this article by yourself :) by creating a file for the article.
+  You can use this tool http://siteconfig.fivefilters.org/.
+
+How can I try to re-fetch this article?
+---------------------------------------
+
+If wallabag failed when fetching an article, you can click on the reload button
+(the third on the below picture).
+
+.. image:: ../../img/user/refetch.png
+   :alt: Refetch content
+   :align: center