weechat: add hint in documentation

doc/package-notes.xml

@@ -721,5 +721,15 @@ plugin's <literal>withPackages</literal> attribute:
 }
 </programlisting>
 </para>
+<para>
+In order to also keep all default plugins installed, it is possible to use
+the following method:
+<programlisting>weechat.override { configure = { availablePlugins, ... }: {
+  plugins = builtins.attrValues (availablePlugins // {
+    python = availablePlugins.python.withPackages (ps: with ps; [ pycrypto python-dbus ]);
+  });
+}; }
+</programlisting>
+</para>
 </section>
 </chapter>