Add a warning not saying that Read the Docs is required

Author: humitos

docs/installation.rst

@@ -34,7 +34,16 @@ To do this, add this extension to your Sphinx's extensions in the ``conf.py`` fi
 After installing the package and adding the extension in the ``conf.py`` file,
 you can use ``:hoverxref:`` role to show tooltip when hovering with the mouse.
 
+.. warning::
+
+   This extension **requires a backend server** to work.
+   Currently only `Read the Docs`_ is supported,
+   so you need to host your documentation on Read the Docs for the extension to work properly.
+
 If you prefer to apply this behavior to *all* your ``:ref:`` in your documentation,
 you can use the config :confval:`hoverxref_auto_ref`.
 
 See :ref:`usage:usage` for more use cases and :ref:`configuration:configuration` for a full list of available configs.
+
+
+.. _Read the Docs: https://readthedocs.org/