README: add common issues

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

README.rst

@@ -41,6 +41,31 @@ dependencies.
 - PIP: `poezio-omemo`
 - Manual: `python3 setup.py install`
 
+Common issues
+-------------
+
+This plugin is **NOT** to be placed in the Poezio plugin folder, doing
+so may shadow the OMEMO library and render it inaccessible from Poezio.
+This module declares itself via `pkg_resources` under the
+`poezio_plugins` group.
+
+Other possible issues when loading the plugin may be that the OMEMO
+library is incorrectly setup.
+
+In a Python interpreter::
+
+  >>> # Is the backend OMEMO library is reachable? (success: no error, no output)
+  >>> import omemo
+  >>> # Is poezio_omemo reachable? (success: no error, no output)
+  >>> import poezio_omemo
+  >>> # Is the module probably declared in plugin entries? (success: true)
+  >>> import pkg_resources
+  >>> 'omemo' in map(lambda e: e.name, pkg_resources.iter_entry_points('poezio_plugins'))
+
+If this doesn't yield any error and Poezio still can't load the plugin,
+try starting it with a debug file (`-d poezio.log`) and join our
+`channel <xmpp:poezio@muc.poez.io?join>`_.
+
 Use in poezio
 -------------