Add documentation to know how to contribute to the documentation :trollface:

docs/en/developer/documentation.rst

@@ -0,0 +1,12 @@
+Contribute to this documentation
+================================
+
+Sources of our documentation are here https://github.com/wallabag/wallabag/tree/master/docs
+
+We use `ReadTheDocs
+<https://readthedocs.org>`__ to generate it.
+
+Pages are written in `Restructured Text
+<https://en.wikipedia.org/wiki/ReStructuredText>`__ format. You can use online tools like http://rst.aaroniles.net/ or http://rst.ninjs.org/ to preview your articles. 
+
+If you create a new page, don't forget to edit the `index.rst <https://raw.githubusercontent.com/wallabag/wallabag/master/docs/en/index.rst>`__ file to add a link in the sidebar.