Add references to libsignal setup to features page

Author: guusdk

docs/source/features.rst

@@ -35,6 +35,12 @@ End to end message encryption (`XEP-0384 OMEMO <https://xmpp.org/extensions/xep-
     of uploaded files. Files will be uploaded WITHOUT ENCRYPTION, even when
     OMEMO is enabled.
 
+.. note::
+    For end-to-end encryption via OMEMO, you'll need to load `libsignal-protocol.js
+    <https://github.com/signalapp/libsignal-protocol-javascript>`_ separately in
+    your page. Take a look at the section on :ref:`libsignal <dependency-libsignal>` and the
+    :ref:`security considerations around OMEMO <feature-omemo>`.
+
 Converse supports OMEMO encryption based on the
 `Signal Protocol <https://github.com/signalapp/libsignal-protocol-javascript>`_.